feat(room_preview): Use room directory as another data source

[jmartinesp]

For getting a RoomPreview we have the following paths, unfolding from Client::get_room_preview.

• If the room is known, get the preview from its RoomInfo.
• If it's not known:
  • Try using MSC3266 to get a preview.
  • If MSC3266 is not enabled, try asking your homeserver for some state and room member data.

The 'not known' part may fail oftentimes if the MSC is not enabled, so we're trying to complement it with the room directory search for public rooms, which will be added as another possible fallback.

The way this works is:

• If we have a room alias, use it for the search, take the first 20 results and look for a room with the expected room id.
• If we don't have it, request 100 rooms and try to find a matching room as a best effort (this may work for small HS, but definitely not for large ones).

In case we have several via parameters, we'll start a search in each of them and stop as soon as one of them returns valid results.

• Public API changes documented in changelogs (optional)

Signed-off-by:

[codecov]

Codecov Report

Attention: Patch coverage is 73.01587% with 17 lines in your changes missing coverage. Please review.

Project coverage is 84.98%. Comparing base (232391c) to head (95083a9).

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/test_utils/mocks.rs	59.25%	11 Missing ⚠️
crates/matrix-sdk/src/room_preview.rs	83.33%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4272      +/-   ##
==========================================
- Coverage   85.00%   84.98%   -0.02%     
==========================================
  Files         274      274              
  Lines       29952    30013      +61     
==========================================
+ Hits        25460    25508      +48     
- Misses       4492     4505      +13     

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