[Task] Fetch missing profiles for all users in a room.

Holding issue for now, requires some new endpoints based on MSC4536 before it can be worked on.

Further details in the parent issue, but TL;DR
- When calling `Room::members()` add some additional logic to detect which members don't have global profiles.
- Call the new endpoint for those members and store their profiles.
- /sync should keep these profiles up to date (or remove them) automatically.