Reviewer Mike - Rename forward_* to send_* and add create_response function

As discussed, this API allows AppServers to create and send responses based on requests, including provisional ones.

UTs updated to test the new function.
I was basing the code on the original version of `reject` which used a `std::string`.  I thought in general we used `std::` objects everywhere and converted to `pj_` objects at the last minute?

I'm happy(ish) to change this now as I'm already going to be adding the `_rx_/_tx_` functions, but I think we should be trying to stabilize the API where we can (especially as the API is mirrored in two places across two repositories).