feat: allow exponentiation post agg for log-fold-change in rank_genes_groups

[ilan-gold]

My two questions here:

1. What to do about t-tests? Do people expect fold change of the input to the t-test (as is currently done) or fold change of the exponent?
2. How do we test this? Dummy groups? Wait for illico to compare?

• Closes rank_genes_groups logfoldchange different than seurat #864
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

❌ Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.04%. Comparing base (9bc2c1e) to head (c2f3738).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/scanpy/tools/_rank_genes_groups.py	71.42%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4037      +/-   ##
==========================================
- Coverage   78.51%   78.04%   -0.47%     
==========================================
  Files         117      117              
  Lines       12753    12764      +11     
==========================================
- Hits        10013     9962      -51     
- Misses       2740     2802      +62     

Flag	Coverage Δ
hatch-test.low-vers	77.79% <73.33%> (-0.02%)	⬇️
hatch-test.pre	57.08% <73.33%> (-7.76%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/scanpy/_settings/presets.py	91.17% <100.00%> (+0.08%)	⬆️
src/scanpy/tools/_rank_genes_groups.py	91.92% <71.42%> (-0.92%)	⬇️

... and 2 files with indirect coverage changes