Room member list improvement

[manuroe]

We are hitting performance issues and features limitation with the current implementation of room members. This issues list all the features we would need now and in a coming future.

• Limit the number of FFI calls (ffi: aggregate RoomMember fields early instead of requiring calling getters #3172) is a prerequesite
• Pagination to improve performance and be O(1) with the number of members
• Observability. The list must update and notify the app on any change like new member, member profile change, etc
• Filtering. Being able to filter on the power level. The user story behind is the app may display the admin users separately
• Sorting based on power level or display name. It can also be a combination of the two. See Element Web for example
• Search based on member display name and/or avatar

This is very similar to all the functionalities of the Room List API.