Add a dev system to accept trigger messages and pipe them into _request messages

[chrisjhoughton]

Sniper intercepts webhook triggers and maps them into a format that goes to the connector.

Currently if you want to test trigger connectors end-to-end offline then you can't with ngrok. It'd be great if you could.

Seems like this basically needs a proxy system in dev mode to route data to the connector in the same way sniper does.