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.AspNetCore/HttpServerTransportOptions.cs

@@ -21,4 +21,17 @@ public class HttpServerTransportOptions
     /// This is useful for running logic before a sessions starts and after it completes.
     /// </summary>
     public Func<HttpContext, IMcpServer, CancellationToken, Task>? RunSessionHandler { get; set; }
+
+    /// <summary>
+    /// Represents the duration of time the server will wait between any active requests before timing out an
+    /// MCP session. This is checked in background every 5 seconds. A client trying to resume a session will
+    /// receive a 404 status code and should restart their session. A client can keep their session open by
+    /// keeping a GET request open. The default value is set to 2 minutes.
+    /// </summary>
+    public TimeSpan IdleTimeout { get; set; } = TimeSpan.FromMinutes(2);
+
+    /// <summary>
+    /// Used for testing the <see cref="IdleTimeout"/>.
+    /// </summary>
+    public TimeProvider TimeProvider { get; set; } = TimeProvider.System;
 }