Allow to observe/listen to room membership state changes for the current user

All clients would probably benefit from having a mechanism to listen to or observe membership state changes for the current user, to understand when it should no longer have access to a room. While a general mechanism to observe room changes (title, topic, power level, members, etc.) would be great, it's probably membership that's the most urgent.

Listening to events in the timeline doesn't seem to work, and neither does observing sliding sync changes, or at least I couldn't make it work.

Start by tracing how the SDK currently exposes timeline events and sliding sync changes, since the issue reports that neither reliably reveals current-user membership changes. Define the public observation mechanism and verify that it reports when the user loses access to a room; the issue does not name specific files or tests.

Written by the indexing model from the issue text.