Add Redis connection factory and manager

`IRedisConnectionManager` is a singleton service responsible for
initialising the connection to the Redis cluster and recreating it in
the rare cases [1] where it fails to reconnect after a transient
connection failure.

It is based heavily on the `RedisConnection` class in the Azure Cache
for Redis sample for ASP.NET Core [2] with a few notable differences:

- `IRedisConnectionManager` is not responsible for providing any retry
  logic. Instead, consumers are expected to call
  `IRedisConnectionManager.CheckException` when an exception occurs
  making a call to Redis. This return value indicates whether the
  exception could have been caused by a transient connection error,
  which the caller can use to decide whether to perform a retry.
- I've added separate configuration options
  (`DroppedConnectionGracePeriod` and `DroppedConnectionEpisodeTimeout`)
  for the two conditions checked against `RedisErrorThreshold` in the
  Azure Cache for Redis sample
- `RedisConnectionManager` implements `IAsyncDisposable` instead of
  `IDisposable`

I've abstracted the creation of Redis connections into a separate
`IRedisConnectionFactory` service purely to facilitate unit testing of
`RedisConnectionManager`. This interface is not intended to be consumed
directly and is therefore internal to the assembly.

[1] StackExchange/StackExchange.Redis#559
[2] https://github.com/Azure-Samples/azure-cache-redis-samples/blob/main/quickstart/aspnet-core/ContosoTeamStats/RedisConnection.cs

Author: smfeest

Buttercup.sln

@@ -34,6 +34,10 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Buttercup.Application", "sr
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Buttercup.Application.Tests", "src\Buttercup.Application.Tests\Buttercup.Application.Tests.csproj", "{00B87344-63DF-4574-A5F6-F1BD5232033F}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Buttercup.Redis", "src\Buttercup.Redis\Buttercup.Redis.csproj", "{E89C44A4-FA47-4EEE-A558-9EAE2944C5AC}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Buttercup.Redis.Tests", "src\Buttercup.Redis.Tests\Buttercup.Redis.Tests.csproj", "{434E5794-262A-45AA-A972-BC43E41D5E15}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -215,6 +219,30 @@ Global
 		{00B87344-63DF-4574-A5F6-F1BD5232033F}.Release|x64.Build.0 = Release|Any CPU
 		{00B87344-63DF-4574-A5F6-F1BD5232033F}.Release|x86.ActiveCfg = Release|Any CPU
 		{00B87344-63DF-4574-A5F6-F1BD5232033F}.Release|x86.Build.0 = Release|Any CPU
+		{E89C44A4-FA47-4EEE-A558-9EAE2944C5AC}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{E89C44A4-FA47-4EEE-A558-9EAE2944C5AC}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{E89C44A4-FA47-4EEE-A558-9EAE2944C5AC}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{E89C44A4-FA47-4EEE-A558-9EAE2944C5AC}.Debug|x64.Build.0 = Debug|Any CPU
+		{E89C44A4-FA47-4EEE-A558-9EAE2944C5AC}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{E89C44A4-FA47-4EEE-A558-9EAE2944C5AC}.Debug|x86.Build.0 = Debug|Any CPU
+		{E89C44A4-FA47-4EEE-A558-9EAE2944C5AC}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{E89C44A4-FA47-4EEE-A558-9EAE2944C5AC}.Release|Any CPU.Build.0 = Release|Any CPU
+		{E89C44A4-FA47-4EEE-A558-9EAE2944C5AC}.Release|x64.ActiveCfg = Release|Any CPU
+		{E89C44A4-FA47-4EEE-A558-9EAE2944C5AC}.Release|x64.Build.0 = Release|Any CPU
+		{E89C44A4-FA47-4EEE-A558-9EAE2944C5AC}.Release|x86.ActiveCfg = Release|Any CPU
+		{E89C44A4-FA47-4EEE-A558-9EAE2944C5AC}.Release|x86.Build.0 = Release|Any CPU
+		{434E5794-262A-45AA-A972-BC43E41D5E15}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{434E5794-262A-45AA-A972-BC43E41D5E15}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{434E5794-262A-45AA-A972-BC43E41D5E15}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{434E5794-262A-45AA-A972-BC43E41D5E15}.Debug|x64.Build.0 = Debug|Any CPU
+		{434E5794-262A-45AA-A972-BC43E41D5E15}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{434E5794-262A-45AA-A972-BC43E41D5E15}.Debug|x86.Build.0 = Debug|Any CPU
+		{434E5794-262A-45AA-A972-BC43E41D5E15}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{434E5794-262A-45AA-A972-BC43E41D5E15}.Release|Any CPU.Build.0 = Release|Any CPU
+		{434E5794-262A-45AA-A972-BC43E41D5E15}.Release|x64.ActiveCfg = Release|Any CPU
+		{434E5794-262A-45AA-A972-BC43E41D5E15}.Release|x64.Build.0 = Release|Any CPU
+		{434E5794-262A-45AA-A972-BC43E41D5E15}.Release|x86.ActiveCfg = Release|Any CPU
+		{434E5794-262A-45AA-A972-BC43E41D5E15}.Release|x86.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(NestedProjects) = preSolution
 		{C1FBF99E-812B-4376-B483-73F708864933} = {309ABF0B-35CC-40DE-BCDF-E326309442D1}
@@ -231,5 +259,7 @@ Global
 		{6984E338-14FA-421E-90B6-921AD05940EF} = {309ABF0B-35CC-40DE-BCDF-E326309442D1}
 		{EE8E0C19-A969-4CEF-AB99-5C41A6B27990} = {309ABF0B-35CC-40DE-BCDF-E326309442D1}
 		{00B87344-63DF-4574-A5F6-F1BD5232033F} = {309ABF0B-35CC-40DE-BCDF-E326309442D1}
+		{E89C44A4-FA47-4EEE-A558-9EAE2944C5AC} = {309ABF0B-35CC-40DE-BCDF-E326309442D1}
+		{434E5794-262A-45AA-A972-BC43E41D5E15} = {309ABF0B-35CC-40DE-BCDF-E326309442D1}
 	EndGlobalSection
 EndGlobal

src/Buttercup.Redis.Tests/Buttercup.Redis.Tests.csproj

@@ -0,0 +1,23 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <RootNamespace>Buttercup.Redis</RootNamespace>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Extensions.TimeProvider.Testing" Version="9.2.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.13.0" />
+    <PackageReference Include="Moq" Version="4.20.72" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.0.2">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+    <PackageReference Include="coverlet.collector" Version="6.0.4">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\Buttercup.Redis\Buttercup.Redis.csproj" />
+  </ItemGroup>
+</Project>

src/Buttercup.Redis.Tests/RedisConnectionFactoryTests.cs

@@ -0,0 +1,29 @@
+using Microsoft.Extensions.Options;
+using StackExchange.Redis;
+using Xunit;
+
+namespace Buttercup.Redis;
+
+public sealed class RedisConnectionFactoryTests
+{
+    #region NewConnection
+
+    [Fact]
+    public async Task NewConnection_ReturnsNewConnection()
+    {
+        var options = new RedisConnectionOptions
+        {
+            ConnectionString = "localhost,abortConnect=false",
+        };
+
+        var connectionFactory = new RedisConnectionFactory(Options.Create(options));
+
+        var connection = await connectionFactory.NewConnection();
+
+        Assert.Equal(
+            ConfigurationOptions.Parse(options.ConnectionString).ToString(),
+            connection.Configuration);
+    }
+
+    #endregion
+}