[Story] Fetch status for all users in the room

### Description

* As a user
* When I join a new rooms
* Then I want to see an up to date status for all users in that room
* So that I would not be mislead

This is a bug fix to something that was discovered during testing rather than an explicit story that needs to be written up, thus the above simply illustrates the issue that when Alice joins a room, she only sees Bob status if she has other shared rooms with Bob or if Bob has recently changed their status (e.g. last 10 updates in the room). To be precise - currently this is an issue only on mobile, however web is using a workaround and needs to be changed as well to make sure it does not overload the server.

The technical solution for this is a dedicated endpoint to pull/fetch the missing profile information that is optimized for this scenario from a performance perspective. A dedicated MSC is written, then implemented by Synapse and used by both mobile and web.

# Acceptance criteria
- The status of all users in the room are up to date after joining a room

# Dependencies
- None

# Out of scope
- Nothing

# Questions
- [x] None

# Subtasks

### Android
-

### iOS
-

### Rust
-

### Other
-

No files or tests are named. Start by reading the dedicated MSC and its Synapse implementation, then review how the mobile and web clients currently obtain room-user status. The work is done when both clients use the optimized endpoint and all users' statuses are up to date after joining a room.

Written by the indexing model from the issue text.