WebRTC: Use Authorization header as standard authentication

[jmelancongen]

Currently, the signaling protocol specifies initial authentication as a mandatory register message when opening the websocket. This forces a signaling server to accept unauthenticated websockets and leave them open for some time to receive the actual authentication information which open us to some DoS/resource exhaustion attacks.

Since we have a precedent in the Uplink specification of authenticating a WebSocket connection via the header or query parameter (to accomodate browser websockets, which can't provide headers). This ensures we use a common way of authenticating these kind of cloud connections in both specs, and allow us to validate that the request is legitimate before accepting any websocket session.

[jmelancongen]

Updated to only add the Authorization header requirement, without removing the register command as discussed during F2F review