Add a debug endpoint

[atc0005]

This endpoint would take no action, but instead echos out a parsed version of a received payload. My recent thinking was that we could exclusively use the atc0005/bounce application for this purpose, but that shows everything received and not the final data structure generated from parsing a received payload.

I'm not sure yet whether this endpoint would need to be protected by a different means than GH-18 is intended to provide.