room preview: allow passing through a list of servers to discover a room with MSC3266

[bnjbvr]

Fixes #3395.

[bnjbvr]

Note to self: since the via parameters are only meaningful when using a room id, rework so that there are two methods:

• get_room_preview_with_id(room_id, via)
• get_room_preview_with_alias(room_alias)

And ditto for the join_room methods.

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 82.96%. Comparing base (83427b3) to head (c9c4329).
Report is 7 commits behind head on main.

Files	Patch %	Lines
crates/matrix-sdk/src/client/mod.rs	66.66%	2 Missing ⚠️
crates/matrix-sdk/src/room_preview.rs	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3407      +/-   ##
==========================================
- Coverage   82.96%   82.96%   -0.01%     
==========================================
  Files         245      245              
  Lines       24901    24905       +4     
==========================================
+ Hits        20660    20663       +3     
- Misses       4241     4242       +1     

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[bnjbvr]

Requesting another round of review because there's been some dramatic changes:

• passing a via list of servers to help resolving is only useful when there's a RoomId, not a RoomAliasId; as a result, the FFI method has been split into two methods.
• I suspect MSC3266's implementation will work better if we pass directly a room alias id, instead of 1. resolving the room alias to a room id, 2. and then pass the room alias id. The reasoning is the same as above: if we passed it only a room id, then it would fail because the via list would be empty; if we pass it the RoomAliasId directly, it might resolve it server-side. Your guess as good as mine.
  • this required a bit of a rewrite of the get_room_preview so it takes a RoomOrAliasId parameter, and a few matches here and there to make it work as intended. I didn't add a changelog entry because there hasn't been any release since the introduction of get_room_preview, as far as I can tell.

[Hywan]

I like the split and I believe it brings clarity, thanks!

[Hywan]

approving