Add support for force relay option

[fatduckling]

Hi

Would it be possible to add an option to force all traffic to go through a relay (TURN server) for added privacy? This allows hiding the local IP address.

Libnice has something similar
https://libnice.freedesktop.org/libnice/NiceAgent.html#NiceAgent--force-relay

This can also be used to test if our TURN servers are configured correctly.

Thanks very much @paullouisageneau