Does a thirdparty protocol lookup to the homeserver to get the
corresponding native/virtual user for a matrix ID. Stores the
mappings in room account data. Involves some slightly nasty workarounds
for that fact that room account data has no local echo.
The accompanying element-web PR with the config documentation should
explain what this is & why. Internally, this breaks the assumption
that call.roomId is the room that the call appears in for the user.
call.roomId may now be a 'virtual' room while the react SDK actually
displays it in a different room. React SDK always stores the calls
under the user-facing rooms, and provides a function to get the
user-facing room for a given call.
