Add backwards compatibility for ACCEPT_RAW, ACCEPT_JSON, and HIDDEN_ENDPOINT

Still needs some docs and a bit more work, proof of concept to see if this is a reasonable way to handle it.

My thought is that endpoints using the new form will access the data with $request->get_body or $request->get_json instead of expecting it to be in $args['data'] based on accept_raw or accept_json
This is crazy and I love it.