uniform color handling between labels and points/shapes

[Sonja-Stockhaus]

As mentioned in #478, here we adapt the color handling of labels so that it works the same way as points/shapes (e.g. you can pass any color like...)

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 83.83%. Comparing base (d22d3b8) to head (e8a940e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #497      +/-   ##
==========================================
+ Coverage   83.80%   83.83%   +0.03%     
==========================================
  Files           8        8              
  Lines        1976     1980       +4     
==========================================
+ Hits         1656     1660       +4     
  Misses        320      320              

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/basic.py	89.32% <100.00%> (ø)
src/spatialdata_plot/pl/render.py	92.51% <100.00%> (ø)
src/spatialdata_plot/pl/render_params.py	89.20% <100.00%> (+0.06%)	:arrow_up:
src/spatialdata_plot/pl/utils.py	78.37% <100.00%> (+0.05%)	:arrow_up:

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