Log candidates for calls
First attempt at this was sending everything from
RTCPeerConnection.getStats() as a separate item in the rageshake,
but the rageshake server doesn't handle that in a particularly sensible
way and it's probably better to pick & choose what data we want explicitly
from a privacy PoV. This summarises the candidates used for the calls into
the log that will be included in rageshakes so we can diagnose connectivity
issues from rageshakes.
Requires https://github.com/matrix-org/matrix-js-sdk/pull/1584
