Extending gex_only option to Visium function

[alexanderchang1]

Extended gex_only function to visium

• Closes Can't read in protein antibody capture data #3113
• Tests included or not required because: It's a straightforward argument pass from visium to read_10x_h5

• Release notes not necessary because: not a big change

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.95%. Comparing base (be99b23) to head (33d2591).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3278   +/-   ##
=======================================
  Coverage   76.95%   76.95%           
=======================================
  Files         109      109           
  Lines       12466    12466           
=======================================
  Hits         9593     9593           
  Misses       2873     2873           

Files with missing lines	Coverage Δ
src/scanpy/readwrite.py	77.53% <100.00%> (ø)