faster reading of h5ad file (~18X faster)

[kaushalprasadhial]

This PR is for faster reading of h5ad file (~18X faster).

	Time
Original	74.79
Updated	4.09
Speedup	18.27

Testing was done in AWS m7i.24xlarge

• Closes #
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

Attention: Patch coverage is 30.21583% with 97 lines in your changes missing coverage. Please review.

Project coverage is 58.27%. Comparing base (fa38823) to head (89c6c72).

Files with missing lines	Patch %	Lines
src/scanpy/readwrite.py	30.21%	97 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3365       +/-   ##
===========================================
- Coverage   75.36%   58.27%   -17.10%     
===========================================
  Files         112      112               
  Lines       13219    13355      +136     
===========================================
- Hits         9963     7783     -2180     
- Misses       3256     5572     +2316     

Files with missing lines	Coverage Δ
src/scanpy/readwrite.py	63.94% <30.21%> (-10.77%)	⬇️

... and 48 files with indirect coverage changes