Feature/outline color for labels

[MeyerBender]

This addresses two previous issues with rendering labels:

1. Users can now set the color of the outline with the outline_color argument, similar to how they could in render_shapes(), see Ability to change color of the border of labels #462
2. Fixes color leak when plotting outlines only in render_labels, see Color leak when outline_alpha=1 and full_alpha=0 #461

Example code to see both changes in action:

import spatialdata as sd
import spatialdata_plot
from spatialdata.datasets import blobs

sdata = blobs()
sdata.pl.render_images('blobs_image', channel=0).pl.render_labels('blobs_labels', outline_alpha=1, fill_alpha=0, outline_color='white').pl.show()

[MeyerBender]

@timtreis Naive question but I can't find where to download the artifacts to fix the tests, is this a permission issue or am I missing something?