Resurrect ServerAgent for external tool access

[fluffyfreak]

This is not a new feature but after a discussion on the Discord channel I did promise to take a look at it and to stop me forgetting (again) I'm going to track this here as a WIP 😁

Todo:

• this code is NOT equivalent

-		Json::FastWriter writer;
-		req.buffer = writer.write(req.data);
+		req.buffer = req.data;

• responding to success is commented out due to me not understanding the JSON changes yet
• once the above is fixed it will need testing