IO for nullable string arrays

[flying-sheep]

• Closes Nullable string columns #679
• Tests added
• Release note added (or unnecessary)

What to use as backend?

• Numcodecs talks about dataframe stuff here: Dataframe codec zarr-developers/numcodecs#452

[codecov]

Codecov Report

Attention: Patch coverage is 98.73418% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.38%. Comparing base (17222d4) to head (d8297a0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/anndata/_io/specs/methods.py	96.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1558      +/-   ##
==========================================
- Coverage   86.72%   84.38%   -2.34%     
==========================================
  Files          37       37              
  Lines        5982     6027      +45     
==========================================
- Hits         5188     5086     -102     
- Misses        794      941     +147     

Files with missing lines	Coverage Δ
src/anndata/_io/zarr.py	83.54% <ø> (ø)
src/anndata/_settings.py	93.83% <100.00%> (+0.04%)	⬆️
src/anndata/tests/helpers.py	83.12% <100.00%> (-10.99%)	⬇️
src/anndata/_io/specs/methods.py	88.67% <96.77%> (+0.27%)	⬆️

... and 6 files with indirect coverage changes

[ilan-gold]

Didn't we want this as opt-in? #679 (comment)

[flying-sheep]

yup, you’re right, I should have requested a review after adding that part