Configurable VFS for matrix_sdk_sqlite compiled to WASM targets

[NoManColonies]

Split from (#6329)

This PR adds support for configuring different VFS backends when matrix_sdk_sqlite is compiled for WASM targets.

Summary of changes

• Change default VFS to in-memory (instead of OPFS)
• Add vfs-opfs-sahpool feature to select OPFS as the VFS backend
• Add vfs-relaxed-idb feature to select IndexedDB as the VFS backend
• vfs-opfs-sahpool and vfs-relaxed-idb are mutually exclusive
• Update cargo xtask ci to run the test suite for each configurable VFS

Notes

• Defaulting to in-memory avoids dedicated worker context assumptions for WASM users
• OPFS and IndexedDB can be explicitly enabled via feature flags

---

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

Signed-off-by: Geng Kongpop geng@finnomena.com

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.85%. Comparing base (85b8c83) to head (33660e2).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-sqlite/src/crypto_store.rs	0.00%	0 Missing and 1 partial ⚠️
crates/matrix-sdk-sqlite/src/state_store.rs	75.00%	0 Missing and 1 partial ⚠️
crates/matrix-sdk-sqlite/src/utils.rs	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6365      +/-   ##
==========================================
- Coverage   89.88%   89.85%   -0.04%     
==========================================
  Files         376      376              
  Lines      103093   103097       +4     
  Branches   103093   103097       +4     
==========================================
- Hits        92670    92637      -33     
- Misses       6863     6884      +21     
- Partials     3560     3576      +16     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing NoManColonies:configurable-vfs (33660e2) with main (85b8c83)}