Add support for gamma parameter in MMD distance

[Zethson]

Fixes #823

1. Now allows for:

import pertpy as pt
adata = pt.dt.distance_example()
dist = pt.tl.Distance(metric="mmd")
df = dist.pairwise(adata, groupby="perturbation", gamma=0.5)

2. Improves typing for Metrics

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 33.33333% with 6 lines in your changes missing coverage. Please review.

Project coverage is 72.58%. Comparing base (45a9266) to head (9ab9ba1).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pertpy/tools/_distances/_distances.py	25.00%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #825   +/-   ##
=======================================
  Coverage   72.58%   72.58%           
=======================================
  Files          47       47           
  Lines        5540     5541    +1     
=======================================
+ Hits         4021     4022    +1     
  Misses       1519     1519           

Files with missing lines	Coverage Δ
pertpy/tools/_coda/_base_coda.py	56.73% <ø> (ø)
pertpy/tools/_distances/_distance_tests.py	93.96% <100.00%> (ø)
pertpy/tools/_distances/_distances.py	90.27% <25.00%> (+0.01%)	⬆️

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