Improve progress reporting (#145)

* Improve progress reporting

- Enable injecting an `IProgress<>` into a tool call to report progress when the client has supplied a progress token
- Add a custom type for ProgressToken and ProgressNotification to map to the schema
- Simplify RequestId to match ProgressToken's shape
- Serialize StdioClientTransport's SendMessageAsync implementation
- Add strongly-typed names for all request methods
- Change all request IDs created by McpJsonRpcEndpoint to include a guid for that endpoint

* Address PR feedback

tests/ModelContextProtocol.Tests/Utils/TestServerTransport.cs

@@ -39,9 +39,9 @@ public class TestServerTransport : ITransport
         SentMessages.Add(message);
         if (message is JsonRpcRequest request)
         {
-            if (request.Method == "roots/list")
+            if (request.Method == RequestMethods.RootsList)
                 await ListRoots(request, cancellationToken);
-            else if (request.Method == "sampling/createMessage")
+            else if (request.Method == RequestMethods.SamplingCreateMessage)
                 await Sampling(request, cancellationToken);
             else
                 await WriteMessageAsync(request, cancellationToken);