Add server-side Streamable HTTP transport support (#330)

- Changes IJsonRpcMessage to an abstract base class so RelatedTransport will always be available
- Streamable HTTP supports multiple concurrent HTTP request with their own indpendent SSE response streams
- RelatedTransport indicates the source or destination of the JsonRpcMessage

- Changes the default RequestId to a JSON number for better compatibility with MCP servers in the wild

tests/ModelContextProtocol.Tests/Protocol/NotificationHandlerTests.cs

@@ -33,7 +33,7 @@ public class NotificationHandlerTests : ClientServerTestBase
             }
         }
 
-        Assert.Equal(Iterations, counter);  
+        Assert.Equal(Iterations, counter);
     }
 
     [Fact]