Spec for transferring VoIP Calls (SPEC-150)

Both sides will have to support this for it to work so it's going to suck if we get clients that don't support it: we should spec it sooner rather than later.

Requirements:
- Between different devices (same person)
- Same device moving networks
- Transferring a call switchboard style in-house
- Possibly: just redirecting a call somewhere else entirely?

Some of these would be just transferring the media (ie. ICE renegotiation). Others would involve moving signalling too.

For just moving media, it would be nice to keep this as simple as possible, possibly just telling the client to start ICE over again.

(Imported from https://matrix.org/jira/browse/SPEC-150)

(Reported by @dbkr)

No files, tests, or entry points are named. Start by resolving the transfer scenarios in the requirements and determining which cases move media, signalling, or both. Done means an agreed Matrix specification for supported call-transfer behavior, including how ICE renegotiation is handled.

Written by the indexing model from the issue text.