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/ITransport.cs

@@ -0,0 +1,27 @@
+using System.Threading.Channels;
+using McpDotNet.Protocol.Messages;
+
+namespace McpDotNet.Protocol.Transport;
+
+/// <summary>
+/// Represents a transport mechanism for MCP communication.
+/// </summary>
+public interface ITransport : IAsyncDisposable
+{
+    /// <summary>
+    /// Gets whether the transport is currently connected.
+    /// </summary>
+    bool IsConnected { get; }
+
+    /// <summary>
+    /// Channel for receiving messages from the transport.
+    /// </summary>
+    ChannelReader<IJsonRpcMessage> MessageReader { get; }
+
+    /// <summary>
+    /// Sends a message through the transport.
+    /// </summary>
+    /// <param name="message">The message to send.</param>
+    /// <param name="cancellationToken">Token to cancel the operation.</param>
+    Task SendMessageAsync(IJsonRpcMessage message, CancellationToken cancellationToken = default);
+}