perf: do HNSW search with threads of CPU runtime

[BubbleCal]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.66667% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 81.17%. Comparing base (04f3aa8) to head (ca397f7).

Files	Patch %	Lines
rust/lance/src/index/vector/hnsw.rs	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2251      +/-   ##
==========================================
- Coverage   81.20%   81.17%   -0.03%     
==========================================
  Files         187      187              
  Lines       54656    54671      +15     
  Branches    54656    54671      +15     
==========================================
  Hits        44381    44381              
- Misses       7780     7791      +11     
- Partials     2495     2499       +4     

Flag	Coverage Δ
unittests	81.17% <91.66%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

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