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/JsonRpcNotification.cs

@@ -11,12 +11,8 @@ namespace ModelContextProtocol.Protocol.Messages;
 /// They are useful for one-way communication, such as log notifications and progress updates.
 /// Unlike requests, notifications do not include an ID field, since there will be no response to match with it.
 /// </remarks>
-public record JsonRpcNotification : IJsonRpcMessage
+public class JsonRpcNotification : JsonRpcMessage
 {
-    /// <inheritdoc />
-    [JsonPropertyName("jsonrpc")]
-    public string JsonRpc { get; init; } = "2.0";
-
     /// <summary>
     /// Gets or sets the name of the notification method.
     /// </summary>