feat(relay):Audioobserver supported on relaying SFU

[mumuan]

If we relay audio track by ortc api,the sdp has no RTPHeader "urn:ietf:params:rtp-hdrext:ssrc-audio-level", so the relaying sfu can not observer the relayed audio track.

[OrlandoCo]

Yeah, this is something under my radar, but needs discussions, since this would work only if you relay the full session, so peers in relayed streams can get audio levels correctly. So we can either specify the audio level as a Relay option or make a session helper to relay sessions to other SFU, that would help for horizontal scaling.

Edit: Sorry I thought it was an Issue, happy to merge !