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

src/ModelContextProtocol/Server/McpServerExtensions.cs

@@ -26,7 +26,7 @@ public static class McpServerExtensions
         }
 
         return server.SendRequestAsync<CreateMessageResult>(
-            new JsonRpcRequest { Method = "sampling/createMessage", Params = request },
+            new JsonRpcRequest { Method = RequestMethods.SamplingCreateMessage, Params = request },
             cancellationToken);
     }
 
@@ -165,7 +165,7 @@ public static class McpServerExtensions
         }
 
         return server.SendRequestAsync<ListRootsResult>(
-            new JsonRpcRequest { Method = "roots/list", Params = request },
+            new JsonRpcRequest { Method = RequestMethods.RootsList, Params = request },
             cancellationToken);
     }