feat(sdk): add high-level Search helpers + ffi bindings

[bnjbvr]

Starting to extract relevant helpers that can be merged ahead of having search spider all rooms in background.

Part of #5350.

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

[codecov]

Codecov Report

❌ Patch coverage is 95.98765% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.88%. Comparing base (a1157d2) to head (2beab7b).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-ui/src/search/mod.rs	95.97%	5 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6394      +/-   ##
==========================================
+ Coverage   89.87%   89.88%   +0.01%     
==========================================
  Files         378      379       +1     
  Lines      103457   103780     +323     
  Branches   103457   103780     +323     
==========================================
+ Hits        92977    93282     +305     
- Misses       6909     6919      +10     
- Partials     3571     3579       +8     

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[codspeed-hq]

Merging this PR will improve performance by ×2.5

⚡ 1 improved benchmark
✅ 49 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	Restore session [memory store]	276.5 ms	112.2 ms	×2.5

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_{Comparing bnjbvr/ui-search-iterators (2beab7b) with main (a1157d2)}

[bnjbvr]

This PR is in a sufficiently good state that it would allow FFI embedders to try out the search feature, build a search UI and so on. Doesn't do automatic indexation of all content, that problem is currently left as an exercise to the reader/embedder.