Run TURN server to hopefully fix RTC issues

[TheUnlocked]

• Create STUN/TURN server

• Add a POST /api/classroom/[classroomId]/activity/ice endpoint that returns all ICE servers:

interface Response {
    servers: RTCIceServer[],
    ttl?: number;
}

See RTCIceServer

• When WEBRTC_STUN_SERVER_URL is set:
  • Include the stun server in the response
• When WEBRTC_TURN_SERVER_URL and WEBRTC_TURN_SECRET is set:
  • Include the turn server in the response, using WEBRTC_TURN_SECRET as the secret to generate credentials (see this for implementation details).