DTLS 1.3 record layer encoding

[theodorsm]

This issue aims to implement RFC9147 section 4.

[adrianosela]

Hey @theodorsm , is this actively being worked on? Otherwise I'm happy to take it. Or else, can you point me to anything needing attention / assignee? cc:// @JoTurk

[theodorsm]

@adrianosela I have been looking into it, but it's a bit stale now - I want to work on this again very soon.