feat: insert api for HNSW

[jacketsj]

We didn't previously have a way of inserting into HNSW post-build.
There's an important aspect to how this works without affecting performance:

• Always keep the nodes stored in a flat structure
• To do that, we need the number of nodes to be "fixed".
• Add an RwLock to the nodes vec itself, and double its size whenever we overflow with a global lock.
  This will result in a brief slow-down whenever the size overflows, so latency of inserts (and also queries) will very occasionally spike briefly, but with smaller and smaller frequency as the graph size grows.

[github-actions]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.95918% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 79.73%. Comparing base (72b6715) to head (02cf8a4).

Files	Patch %	Lines
rust/lance-index/src/vector/hnsw/builder.rs	97.95%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2395      +/-   ##
==========================================
+ Coverage   79.70%   79.73%   +0.02%     
==========================================
  Files         200      200              
  Lines       54937    55015      +78     
  Branches    54937    55015      +78     
==========================================
+ Hits        43788    43864      +76     
- Misses       8609     8612       +3     
+ Partials     2540     2539       -1     

Flag	Coverage Δ
unittests	79.73% <97.95%> (+0.02%)	⬆️

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