Server support (stdio) (#19)

* Server support (stdio)
* TestServer project
* Refactored McpClient class to extract shared endpoint logic (with McpServer) to a base class, McpJsonRpcEndpoint.
* Improved xmldoc comments for protocol types, mirroring specification schema comments.
* Update README.MD

src/mcpdotnet/Protocol/Transport/IServerTransport.cs

@@ -0,0 +1,13 @@
+namespace McpDotNet.Protocol.Transport;
+
+/// <summary>
+/// Represents a transport mechanism for MCP communication (from the server).
+/// </summary>
+public interface IServerTransport : ITransport
+{
+    /// <summary>
+    /// Starts listening for incoming messages.
+    /// </summary>
+    /// <param name="cancellationToken">Token to cancel the operation.</param>
+    Task StartListeningAsync(CancellationToken cancellationToken = default);
+}