Add missing usings to fix issues when ImplicitUsings not set

Author: cezarypiatek

src/GrpcToRestGenerator/GrpcMockServer.cs

@@ -20,6 +20,8 @@
 using GrpcTestKit.TestConnectors;
 using System;
 using System.Net;
+using System.Threading;
+using System.Linq;
 
 /*MockServerNamespace*/
 

src/GrpcToRestGenerator/StubHelperBuilder.cs

@@ -20,7 +20,12 @@ public string Build(IReadOnlyList<INamedTypeSymbol> proxyBaseClasses)
         {
             {
                 {
+                    _stubBuilder.AppendLine("using System;");
                     _stubBuilder.AppendLine("using GrpcTestKit.TestConnectors;");
+                    _stubBuilder.AppendLine("using System.Threading;");
+                    _stubBuilder.AppendLine("using System.Threading.Tasks;");
+                    _stubBuilder.AppendLine("using System.Linq;");
+                    _stubBuilder.AppendLine("using System.Collections.Generic;");
                     if (string.IsNullOrWhiteSpace(_helperNamespace) == false)
                     {
                         _stubBuilder.AppendLine($"namespace {_helperNamespace};");