Open a WebRTC session

Exchanges an SDP offer for an answer, the way WHIP does. The offer holds one audio transceiver (`sendrecv`) and one data channel labelled `06line`, with every ICE candidate already gathered (no trickle ICE). Use the `ice_servers` returned with the ephemeral key. The session lives as long as the peer connection: it carries the audio of the calls placed or answered with its `id`, one at a time, and pushes `call.incoming`, `call.state` and `line.state` on the data channel. Audio is G.711 µ-law (PCMU), 8 kHz, which every WebRTC stack supports; an offer without PCMU gets no audio. Every session of the company hears `call.incoming` (a session of a key restricted to a line, only for its line), right after `session.ready` for a call already ringing, then `call.state` when the call stops ringing. A session closed with `DELETE` ends its call `hangup`; a session lost on the way (ICE failed) ends it `failed`.

curl -X POST 'https://api.06line.com/v1/webrtc/sessions' \  -H 'Content-Type: application/json' \  -d '{  "sdp": "v=0\r\no=- 4611731400430051336 2 IN IP4 127.0.0.1\r\n..."}'

{  "id": "rtc_01jbq9h7r3m5x1k8t2v6c4d0ya",  "sdp": "v=0\r\no=- 1902398745 2 IN IP4 0.0.0.0\r\n..."}