feat: compat profiles for writing

[flying-sheep]

OK, let’s get this going, I think it’s nicer for users than individual feature gates.

This is a rather low-engineering version:

• no integration with the registry (only settings.override)
• individual features don’t live in the data structure, but rather there’s a version comparison wherever something’s gated on
• therefore the docs are manually written and not autogenerated

we might go with this and refactor as we go or make it more sophisticated from the start, what do you think @ilan-gold?

---

• Closes Compatibility profiles for writing #2057
• Tests added

• Release note not necessary because:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.86%. Comparing base (ad6c4f4) to head (b4ddd7f).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2642      +/-   ##
==========================================
- Coverage   87.60%   85.86%   -1.74%     
==========================================
  Files          49       49              
  Lines        8123     8157      +34     
==========================================
- Hits         7116     7004     -112     
- Misses       1007     1153     +146     

Files with missing lines	Coverage Δ
src/anndata/__init__.py	80.95% <100.00%> (ø)
src/anndata/_core/anndata.py	86.42% <100.00%> (+0.01%)	⬆️
src/anndata/_core/storage.py	95.83% <100.00%> (+0.52%)	⬆️
src/anndata/_io/h5ad.py	92.82% <100.00%> (+0.13%)	⬆️
src/anndata/_io/specs/registry.py	95.02% <100.00%> (+0.13%)	⬆️
src/anndata/_io/utils.py	75.46% <100.00%> (ø)
src/anndata/_io/zarr.py	86.15% <100.00%> (+0.10%)	⬆️
src/anndata/_settings.py	100.00% <100.00%> (ø)

... and 9 files with indirect coverage changes