Bug report

Bug summary
colormaps don't have getters for under, over, bad colors (although the corresponding setters exist).
I am writing a function that accepts a colormap as an argument. I want to ensure that the colormap passed to the function has the same color for the "bad" and "under" conditions. The only way around this is to access "private" attributes cmap._rgba_under, cmap._rgba_bad as shown below.

Code for reproduction

cmap = plt.get_cmap('cividis')
cmap.set_bad('k')
cmap.set_under('k')

assert cmap._rgba_bad == cmap._rgba_under

If adding getters (for under, over, bad colors) for the colormaps sounds like a good improvement, i would be interested in submitting a PR.

Actual outcome

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Expected outcome

Matplotlib version

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