feat(event cache store): add an IndexedDB implementation

[ospfranco]

This PR is WIP.

This PR is meant to add a EventCache store for the the matrix-sdk-indexeddb crate of the SDK meant to be used in WASM contexts. Similar to the sqlite implementation is supposed to provide a disk persisted implementation for storing Events in LinkedChunks.

It follows the general structure of matrix-sdk-indexeddb/src/crypto_store and matrix-sdk-indexeddb/src/state_store, a new module matrix-sdk-indexeddb/src/event_cache_store has been added.

There are still many TODOs and questions in the codebase to which I would appreciate some guidance from the SDK maintainers.

• Public API changes documented in changelogs (optional)

Signed-off-by: Oscar Franco

[ospfranco]

Follows the same pattern as state_store, not sure if this is wanted or not.

[ospfranco]

Will be removed later

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.73%. Comparing base (3f71d9a) to head (d390e07).
Report is 721 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4617      +/-   ##
==========================================
+ Coverage   85.71%   85.73%   +0.01%     
==========================================
  Files         291      291              
  Lines       33323    33322       -1     
==========================================
+ Hits        28563    28568       +5     
+ Misses       4760     4754       -6     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Hywan]

Be aware that #4632 is likely to create a non-negligible conflict with your patches. The new methods to load chunk from the event cache are going to be simpler and will allow to incrementally load the chunks.