mcp: NewClient and NewServer take Implementation (#113)

Change the name and version arguments to NewClient and NewServer to a
`*Implementation`, to future-proof against the spec.

Fixes #109.

mcp/example_progress_test.go

@@ -16,7 +16,7 @@ var nextProgressToken atomic.Int64
 // This middleware function adds a progress token to every outgoing request
 // from the client.
 func Example_progressMiddleware() {
-	c := mcp.NewClient("test", "v1", nil)
+	c := mcp.NewClient(testImpl, nil)
 	c.AddSendingMiddleware(addProgressToken[*mcp.ClientSession])
 	_ = c
 }