feat: add moq-sub to subscribe to media from moq relays

[Frando]

This adds a simple moq-sub tool which allows to connect to a moq relay and subscribe to a broadcast. It expects the format as published by moq-pub, and will then dump the first video and first audio track to stdout. By piping moq-sub into e.g. ffplay you can play media from a moq relay.

Also includes a dev/sub script that works without further arguments when using dev/relay and dev/pub.

Also added an optional AUDIO=1 env var to dev/pub to not skip audio tracks.

[Frando]

Feel free to merge this and my other PR :) I am not working on these further for the time being.

[kixelated]

Working on updating this to the new API.