feat: move hashsolo to sc.pp

[flying-sheep]

Basically just moving the code over and adding an anndata.acc based version of the API on top.

People had issues with our old API since it only supported obs columns:

• https://www.biostars.org/p/9561534/
• https://discourse.scverse.org/t/how-to-install-solo-sc-or-use-hashsolo-from-scvi/1702

The new API of course supports everything! Yay accessors!

• Closes move sc.external.pp.hashsolo into sc.pp #4304
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.06%. Comparing base (6b5405d) to head (ece9b6d).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/scanpy/get/get.py	72.22%	5 Missing ⚠️
src/scanpy/preprocessing/_hashsolo.py	96.11%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4303      +/-   ##
==========================================
+ Coverage   80.01%   80.06%   +0.05%     
==========================================
  Files         132      133       +1     
  Lines       13397    13406       +9     
==========================================
+ Hits        10719    10733      +14     
+ Misses       2678     2673       -5     

Flag	Coverage Δ
hatch-test.low-vers	77.99% <61.90%> (-0.24%)	⬇️
hatch-test.pre	79.93% <92.85%> (+0.05%)	⬆️

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

Files with missing lines	Coverage Δ
src/scanpy/external/pp/_hashsolo.py	100.00% <100.00%> (+11.96%)	⬆️
src/scanpy/get/__init__.py	100.00% <ø> (ø)
src/scanpy/get/_aggregated.py	95.25% <100.00%> (ø)
src/scanpy/preprocessing/__init__.py	100.00% <100.00%> (ø)
src/scanpy/preprocessing/_hashsolo.py	96.11% <96.11%> (ø)
src/scanpy/get/get.py	89.45% <72.22%> (-1.12%)	⬇️

[selmanozleyen]

Some edge cases inherited from main and one new behaviour with the pd.unique and loc usage instead of the old np.unique. Plus one nitpick on dead return variables. But the rest seems good