Refactor transports to help enable graceful shutdown (#142)

- This also paves the way for better multi-session support
- We should definitely rethink names for the transport API
- For now, I kept the names similar as possible, so we can
  focus on the API shape

Co-authored-by: Stephen Toub <stoub@microsoft.com>

tests/ModelContextProtocol.Tests/Configuration/McpServerBuilderExtensionsTransportsTests.cs

@@ -16,7 +16,7 @@ public class McpServerBuilderExtensionsTransportsTests
 
         builder.Object.WithStdioServerTransport();
 
-        var transportType = services.FirstOrDefault(s => s.ServiceType == typeof(IServerTransport));
+        var transportType = services.FirstOrDefault(s => s.ServiceType == typeof(ITransport));
         Assert.NotNull(transportType);
         Assert.Equal(typeof(StdioServerTransport), transportType.ImplementationType);
     }