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

src/ModelContextProtocol/Client/McpClient.cs

@@ -5,6 +5,7 @@ using ModelContextProtocol.Protocol.Types;
 using ModelContextProtocol.Shared;
 using ModelContextProtocol.Utils.Json;
 using System.Text.Json;
+using System.Threading;
 
 namespace ModelContextProtocol.Client;
 
@@ -59,7 +60,7 @@ internal sealed partial class McpClient : McpEndpoint, IMcpClient
 
                 RequestHandlers.Set(
                     RequestMethods.SamplingCreateMessage,
-                    (request, cancellationToken) => samplingHandler(
+                    (request, _, cancellationToken) => samplingHandler(
                         request,
                         request?.Meta?.ProgressToken is { } token ? new TokenProgress(this, token) : NullProgress.Instance,
                         cancellationToken),
@@ -76,7 +77,7 @@ internal sealed partial class McpClient : McpEndpoint, IMcpClient
 
                 RequestHandlers.Set(
                     RequestMethods.RootsList,
-                    rootsHandler,
+                    (request, _, cancellationToken) => rootsHandler(request, cancellationToken),
                     McpJsonUtilities.JsonContext.Default.ListRootsRequestParams,
                     McpJsonUtilities.JsonContext.Default.ListRootsResult);
             }