RTCSession: new event on('sdp') to allow SDP modifications.
RTCSession: new event on('sdp') to allow SDP modifications.

Event data attributes:
- originator
- type
- sdp

- originator='remote' is fired before passing a remote SDP to RTCPeerConnection.
- originator='local' is fired before sending a local SDP.

voxbone/JsSIP,tqcenglish/JsSIP