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/Shared/RequestHandlers.cs

@@ -1,4 +1,5 @@
 using ModelContextProtocol.Protocol.Messages;
+using ModelContextProtocol.Protocol.Transport;
 using ModelContextProtocol.Utils;
 using System.Text.Json;
 using System.Text.Json.Nodes;
@@ -30,7 +31,7 @@ internal sealed class RequestHandlers : Dictionary<string, Func<JsonRpcRequest,
     /// </remarks>
     public void Set<TRequest, TResponse>(
         string method,
-        Func<TRequest?, CancellationToken, ValueTask<TResponse>> handler,
+        Func<TRequest?, ITransport?, CancellationToken, ValueTask<TResponse>> handler,
         JsonTypeInfo<TRequest> requestTypeInfo,
         JsonTypeInfo<TResponse> responseTypeInfo)
     {
@@ -42,7 +43,7 @@ internal sealed class RequestHandlers : Dictionary<string, Func<JsonRpcRequest,
         this[method] = async (request, cancellationToken) =>
         {
             TRequest? typedRequest = JsonSerializer.Deserialize(request.Params, requestTypeInfo);
-            object? result = await handler(typedRequest, cancellationToken).ConfigureAwait(false);
+            object? result = await handler(typedRequest, request.RelatedTransport, cancellationToken).ConfigureAwait(false);
             return JsonSerializer.SerializeToNode(result, responseTypeInfo);
         };
     }