Extend progress notification support (#163)

* Extend progress notification support

* Address feedback and fix test

---------

Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/ModelContextProtocol/IMcpEndpoint.cs

@@ -0,0 +1,35 @@
+using ModelContextProtocol.Protocol.Messages;
+
+namespace ModelContextProtocol;
+
+/// <summary>Represents a client or server MCP endpoint.</summary>
+public interface IMcpEndpoint : IAsyncDisposable
+{
+    /// <summary>Sends a generic JSON-RPC request to the connected endpoint.</summary>
+    /// <typeparam name="TResult">The expected response type.</typeparam>
+    /// <param name="request">The JSON-RPC request to send.</param>
+    /// <param name="cancellationToken">A token to cancel the operation.</param>
+    /// <returns>A task containing the client's response.</returns>
+    Task<TResult> SendRequestAsync<TResult>(JsonRpcRequest request, CancellationToken cancellationToken = default) where TResult : class;
+
+    /// <summary>Sends a message to the connected endpoint.</summary>
+    /// <param name="message">The message.</param>
+    /// <param name="cancellationToken">A token to cancel the operation.</param>
+    Task SendMessageAsync(IJsonRpcMessage message, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Adds a handler for server notifications of a specific method.
+    /// </summary>
+    /// <param name="method">The notification method to handle.</param>
+    /// <param name="handler">The async handler function to process notifications.</param>
+    /// <remarks>
+    /// <para>
+    /// Each method may have multiple handlers. Adding a handler for a method that already has one
+    /// will not replace the existing handler.
+    /// </para>
+    /// <para>
+    /// <see cref="NotificationMethods"> provides constants for common notification methods.</see>
+    /// </para>
+    /// </remarks>
+    void AddNotificationHandler(string method, Func<JsonRpcNotification, Task> handler);
+}