feat: add bbknn

[Intron7]

This PR add a scanpy native bbknn implementation because of the removal of external

fixes #4302

[codecov]

Codecov Report

❌ Patch coverage is 92.50000% with 9 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@e6269f5). Learn more about missing BASE report.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/scanpy/neighbors/_common.py	61.90%	8 Missing ⚠️
src/scanpy/neighbors/_bbknn.py	98.90%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4306   +/-   ##
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  Coverage        ?   80.12%           
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  Files           ?      134           
  Lines           ?    13534           
  Branches        ?        0           
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  Hits            ?    10844           
  Misses          ?     2690           
  Partials        ?        0           

Flag	Coverage Δ
hatch-test.pre	80.12% <92.50%> (?)

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Files with missing lines	Coverage Δ
src/scanpy/_utils/__init__.py	75.79% <ø> (ø)
src/scanpy/external/pp/_bbknn.py	100.00% <100.00%> (ø)
src/scanpy/neighbors/_types.py	100.00% <100.00%> (ø)
src/scanpy/preprocessing/__init__.py	100.00% <100.00%> (ø)
src/scanpy/neighbors/_bbknn.py	98.90% <98.90%> (ø)
src/scanpy/neighbors/_common.py	72.54% <61.90%> (ø)

[flying-sheep]

Looks great!

I deduplicated the transformer creation method and adapted the heuristic a bit, and added a benchmark suite to check it. Take a look if that makes sense or if I missed something!

The only piece I think is missing is that we should get rid of legacy cruft, but I can take care of that:

• no reason for _accepts_legacy_random_state
• things like batch_key and so on should become something accepting AdRefs or strs in the resolve format ("obs.obs_colname" instead of "obs_colname", and then _key of course makes no sense).
• including use_rep: #4315