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/Protocol/Messages/JsonRpcMessageWithId.cs

@@ -0,0 +1,25 @@
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol.Messages;
+
+/// <summary>
+/// Represents a JSON-RPC message used in the Model Context Protocol (MCP) and that includes an ID.
+/// </summary>
+/// <remarks>
+/// In the JSON-RPC protocol, messages with an ID require a response from the receiver.
+/// This includes request messages (which expect a matching response) and response messages
+/// (which include the ID of the original request they're responding to).
+/// The ID is used to correlate requests with their responses, allowing asynchronous
+/// communication where multiple requests can be sent without waiting for responses.
+/// </remarks>
+public abstract class JsonRpcMessageWithId : JsonRpcMessage
+{
+    /// <summary>
+    /// Gets the message identifier.
+    /// </summary>
+    /// <remarks>
+    /// Each ID is expected to be unique within the context of a given session.
+    /// </remarks>
+    [JsonPropertyName("id")]
+    public RequestId Id { get; init; }
+}