EDIT: closes #30511

This PR continues the work of #28658 and #28454, #29876, aiming to close #14168. (Feature request: Bivariate colormapping)

This PR allows Colorizer and ColorizingArtist to work with MultiNorm and BivarColormap and MultivarColormap

i.e. this PR will allow:

import matplotlib.pyplot as plt
import numpy as np
import matplotlib as mpl
a = np.arange(12).reshape(3,4)
b = np.arange(12).reshape(4,3).T

norm = mpl.colors.Normalize(vmin=0, vmax=11)
col = mpl.colorizer.Colorizer(cmap='BiOrangeBlue', norm=[norm, norm])

plt.imshow(col.to_rgba((a, b)))

Features not included in this PR:

• Exposes the functionality provided by MultiNorm together with BivarColormap and MultivarColormap to the plotting functions axes.imshow(...), axes.pcolor(...), and axes.pcolormesh(...)`
• fig.colorbar() equivalents for BivarColormap and MultivarColormap
• Examples in the docs

PR checklist

• new and changed code is tested