Support Opus on SIP Trunks

[ruhnet]

When trying to enable OPUS with "OPUS/48000/2": true in the config file, SIP invites still don't offer the Opus codec over the trunk.

According to the contents of sip/pkg/sip
/media_codecs.go
the only codecs supported on the SIP side of things are G711 and G722.

I looked at the pkg/media/opus/opus.go file and compared it to pkg/media/g722/g722.go and I see that some functions are similar, but the opus.go file is missing the init() function and doesn't register itself.

Many SIP trunks (particularly when connecting to a PBX of some sort) support the Opus codec, so it would be very useful to have it available for use on the SIP side.

[dennwc]

It's definitely on our roadmap, especially because LiveKit uses OPUS internally.