Store SlidingSyncPositionMarkers in EventCacheStore

[mgoldenberg]

Currently, the SlidingSyncPositionMarkers are stored in the CryptoStore. This store was chosen, as the position markers needed to be accessible in a way that could be synchronized across processes - i.e., a store protected by a cross-process lock.

Originally, the only store to provide this functionality was the CryptoStore, but this means that when end-to-end encryption is not enabled, there is no way to store the position markers. So, now that the EventCacheStore is protected by a cross-process lock and is always accessible, it is a more reasonable option.

Changes

• Add functions to the EventCacheStore trait for getting, setting, and removing custom values
  • This mimics the CryptoStore, which makes this pull request simpler - but we could use more tailored functions for this behavior, if that is preferable.
• Implement the above functions for in-memory, SQLite, and IndexedDB implementations of EventCacheStore

---

Closes #6401.

• I've documented the public API Changes in the appropriate CHANGELOG.md files.
• This PR was made with the help of AI.

Signed-off-by: Michael Goldenberg m@mgoldenberg.net

[codecov]

Codecov Report

❌ Patch coverage is 76.59574% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.93%. Comparing base (2ec031b) to head (2f95082).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/event_cache/redecryptor.rs	0.00%	6 Missing ⚠️
crates/matrix-sdk/src/sliding_sync/cache.rs	77.77%	0 Missing and 4 partials ⚠️
crates/matrix-sdk-common/src/cross_process_lock.rs	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6541   +/-   ##
=======================================
  Coverage   89.92%   89.93%           
=======================================
  Files         381      381           
  Lines      105914   105942   +28     
  Branches   105914   105942   +28     
=======================================
+ Hits        95243    95274   +31     
+ Misses       7029     7022    -7     
- Partials     3642     3646    +4     

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

[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

---

_{Comparing mgoldenberg:sliding-sync-pos-in-event-cache-store (2f95082) with main (2ec031b)}