feat: add layer argument to 10x read functions

[flying-sheep]

• Closes Add layer parameter to read_10x_* functions #4071
• Tests included or not required because:

• Release notes not necessary because:

waits for design decision in #4071

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.96%. Comparing base (2fa6ac0) to head (3678191).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4072      +/-   ##
==========================================
- Coverage   78.60%   77.96%   -0.64%     
==========================================
  Files         118      118              
  Lines       12756    12767      +11     
==========================================
- Hits        10027     9954      -73     
- Misses       2729     2813      +84     

Flag	Coverage Δ
hatch-test.low-vers	77.92% <100.00%> (+0.01%)	⬆️
hatch-test.pre	20.75% <40.00%> (-57.74%)	⬇️

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Files with missing lines	Coverage Δ
src/scanpy/_settings/presets.py	87.02% <100.00%> (+0.51%)	⬆️
src/scanpy/readwrite.py	81.08% <100.00%> (-0.21%)	⬇️

... and 8 files with indirect coverage changes