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 29.41176% with 96 lines in your changes missing coverage. Please review.

Project coverage is 58.87%. Comparing base (5654389) to head (bf77dc1).

Files with missing lines	Patch %	Lines
src/scanpy/readwrite.py	29.41%	96 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3365       +/-   ##
===========================================
- Coverage   76.67%   58.87%   -17.80%     
===========================================
  Files         112      112               
  Lines       12946    13079      +133     
===========================================
- Hits         9926     7700     -2226     
- Misses       3020     5379     +2359     

Files with missing lines	Coverage Δ
src/scanpy/readwrite.py	65.25% <29.41%> (-11.63%)	⬇️

... and 49 files with indirect coverage changes