GH-39335: [C#] Support creating FlightClient with Grpc.Core.Channel (#39348)

as well as Grpc.Net.Client.GrpcChannel by changing our constructor arg to Grpc.Core.ChannelBase which both classes inherit from.

### Rationale for this change

### What changes are included in this PR?

Changing the constructor of C#'s Flight Client to take in a ChannelBase which allows for multiple implementations of gRPC channels to be passed in.

### Are these changes tested?

Existing tests already cover the use but have also manually tested in a separate app (

### Are there any user-facing changes?

No as we're just changing the constructor to take in a parent/base class instead.
* Closes: #39335

Authored-by: John Garland <[email protected]>
Signed-off-by: Curt Hagenlocher <[email protected]>

Author: johnnyg

csharp/src/Apache.Arrow.Flight/Client/FlightClient.cs

@@ -16,10 +16,8 @@
 using System.Threading.Tasks;
 using Apache.Arrow.Flight.Internal;
 using Apache.Arrow.Flight.Protocol;
-using Apache.Arrow.Flight.Server;
 using Apache.Arrow.Flight.Server.Internal;
 using Grpc.Core;
-using Grpc.Net.Client;
 
 namespace Apache.Arrow.Flight.Client
 {
@@ -29,7 +27,7 @@ public class FlightClient
 
         private readonly FlightService.FlightServiceClient _client;
 
-        public FlightClient(GrpcChannel grpcChannel)
+        public FlightClient(ChannelBase grpcChannel)
         {
             _client = new FlightService.FlightServiceClient(grpcChannel);
         }