Pass STUN information from Broker to WebRTC Client

The clients may be in filtered regions where common ICE servers may be unavailable.

May need to revise the Configuration interface of go-webrtc while figuring this part out.

\ Migrated from https://github.com/keroserene/snowflake/issues/3

We had some discussion in a recent meeting about the original intent of this ticket.

I think to address the need of "default STUN servers are blocked for the client", it would be more interesting to pursue ways in which NAT punching information for the client could be sent over the domain-fronted connection to the broker. Some discussion on legacy/trac#30579 (moved) laid out reasons why a more private way of getting ICE candidates is desirable.