fix(room): make the room previews more reliable

[bnjbvr]

This should help make room previews more reliable:

• the behavior of Room::history_visibility was to always return an HistoryVisibility value, even when the event is missing. In case it was missing, it would return WorldReadable, which is against the spec, which states that it should be Shared, in that case.
• as noted in ci: test_room_preview intermittently fails #3483 (comment), the m.room.create and m.room.history_visibility MUST be present to correctly calculate the room_type and is_world_readable fields from a RoomPreview.

Solution for 1 consists in splitting the method into two: one that returns an Option<HistoryVisibility>, and another that returns the better sensible default as specified by the spec.

Solution for 2 consists in adding those two events to the required_state list for the room list service. For sync v2, we're getting all the events anyways, so we should handle them correctly.

Fixes #3483.

[bnjbvr]

Latest fixup! commit includes a fix for the integration test; the behavior has slightly changed when we try to sync a room's members while being invited. If we don't know about the history visibility, we now assume it's "shared" instead of "world visible". As a result, we don't even run the request for sync_members(), which means what was a failure before (forward from the synapse server) becomes a soft pass (no request is spawn, sync_members() early returns Ok).

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85.00%. Comparing base (c61f707) to head (3675c40).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-base/src/client.rs	66.66%	2 Missing ⚠️
crates/matrix-sdk-base/src/rooms/normal.rs	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4325      +/-   ##
==========================================
- Coverage   85.07%   85.00%   -0.08%     
==========================================
  Files         275      275              
  Lines       30314    30320       +6     
==========================================
- Hits        25789    25772      -17     
- Misses       4525     4548      +23     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.