event cache: introduce the Paginator API

[bnjbvr]

This introduces a new helper object to run arbitrary pagination requests, backwards- or forward-. At the moment they're disconnected from the event cache, although I've put the files there for future convenience, since at some point we'll want to merge the retrieved events with the cache (? maybe).

This little state machine makes it possible to retrieve the initial data, given an initial event id, using the /context endpoint; then allow stateful pagination using a paginator kind of API. Paginating in the timeline indicates whether we've reached the start/end of the timeline.

The test for the state subscription is quite extensive and makes sure the basic functionality works as intended.

Some testing helpers have been (re)introduced in the SDK crate, simplifying the code, and introducing a better EventFactory / EventBuilder pattern than the existing one in the matrix-sdk-test crate. In particular, this can make use of some types in matrix-sdk, notably SyncTimelineEvent and TimelineEvent, and I've found the API to be simpler to use as well.

Commit history is messy, so we might want to squash when merging.

Part of #3234.

[codecov]

Codecov Report

Attention: Patch coverage is 72.25434% with 48 lines in your changes are missing coverage. Please review.

Project coverage is 83.58%. Comparing base (fcfdaad) to head (f4b63cc).
Report is 15 commits behind head on main.

Files	Patch %	Lines
crates/matrix-sdk/src/test_utils/events.rs	48.14%	28 Missing ⚠️
crates/matrix-sdk/src/event_cache/paginator.rs	81.98%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3309      +/-   ##
==========================================
- Coverage   83.64%   83.58%   -0.06%     
==========================================
  Files         238      240       +2     
  Lines       24662    24835     +173     
==========================================
+ Hits        20629    20759     +130     
- Misses       4033     4076      +43     

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[andybalaam]

This looks great! Lots of questions and suggestions, and an ask for more tests. Thanks!

[bnjbvr]

I've now added isolated tests for getting events from start_from / paginate_backwards / paginate_forwards, and I think it covers all the things you've asked — PTAL!

[andybalaam]

Brilliant, thank you!

[andybalaam]

This is great!