STOMP message headers are not alterable

[adam-vessey]

Looking into adjusting STOMP headers for a couple of reasons (including making things persistent from another module, to address #1857, as well as making some bigger tweaks to authentication); however, the STOMP headers are being set explicitly.

As such, looking to introduce some other mechanism such that they might be manipulated, via hooks or events. Events seems like the most flexible with priorities, and is the mechanism already in play to generate JWTs.