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 92.68293% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.32%. Comparing base (84b6b1b) to head (ad9cfe6).
⚠️ Report is 1 commits behind head on main.
✅ 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	98.93%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4306      +/-   ##
==========================================
+ Coverage   80.14%   80.32%   +0.17%     
==========================================
  Files         133      134       +1     
  Lines       13478    13591     +113     
==========================================
+ Hits        10802    10917     +115     
+ Misses       2676     2674       -2     

Flag	Coverage Δ
hatch-test.low-vers	77.48% <26.01%> (-0.45%)	⬇️
hatch-test.pre	80.20% <92.68%> (+0.18%)	⬆️

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

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%> (+54.54%)	⬆️
src/scanpy/neighbors/__init__.py	81.54% <ø> (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	98.93% <98.93%> (ø)
src/scanpy/neighbors/_common.py	72.54% <61.90%> (+0.94%)	⬆️

... and 1 file with indirect coverage changes

[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

[flying-sheep]

OK, merging #4315 before this one, then all of sc.external is deprecated!