chore(sdk): Rename RoomEventCacheListener to RoomEventCacheSubscriber

[Hywan]

Extracted from #5247.

This patch removes a name ambiguity around listener vs. subscriber.
Both terms are used to talk about RoomEventCacheListener. We usually
use the term subscriber for the type being returned by a subscribe
method. The code refers to this sometimes as listener, sometimes
as subscriber, sometimes both in the same sentence, which can be very
confusing! This patch solves this by using the subscriber term only.

Moreover, the term listener refers to a callback (usually a function) that is
passed to an event-based or reactive system, like we have in matrix_sdk_ffi
for example. It's not appropriate here.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 4 lines in your changes missing coverage. Please review.

Project coverage is 90.20%. Comparing base (25d217c) to head (bb2c520).
Report is 6 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/event_cache/room/mod.rs	78.94%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5269      +/-   ##
==========================================
+ Coverage   90.18%   90.20%   +0.01%     
==========================================
  Files         334      334              
  Lines      104822   104825       +3     
  Branches   104822   104825       +3     
==========================================
+ Hits        94535    94557      +22     
+ Misses       6235     6216      -19     
  Partials     4052     4052              

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[bnjbvr]

lgtm, thanks!