Add custom attributes on BYE method thru X-headers

[flaviogoncalves]

We are integrating livekit with our Call Flow Builder. Using custom attributes we can now select agents and send data to the agent. This agents has tools and call these tools during the interaction to get data. Example: SSN. We can set this data to the SIP participant easily in the agent. What we need is the option to add this data to X-Headers in the end of the call to continue the call flow. If we don't have the information we can't go to the next node without the information obtained in the agent.

The modification we are suggesting is an option to send in the BYE the SIP participant attributes in the form of custom X-attribute headers.

[dennwc]

Interesting use case! Indeed, we currently only pass custom attributes to INVITE or 200 OK, but not on BYE. I'll add a field for it as well: livekit/protocol#893