Fix support for x/y, horizontal/verticalalignment and loc for plt.x/ylabel in WCSAxes

While fixing https://github.com/astropy/astropy/pull/9875 I noticed that we don't properly support the ``x``, ``y``, ``horizontalalignment``, ``verticalalignment`` and ``loc`` arguments to ``plt.xlabel`` and ``plt.pylabel`` in WCSaxes. We should fix this! (I consider this a bug, so we could backport to 4.0.x)

Start by locating WCSAxes handling for plt.xlabel and plt.ylabel, then compare its support for x, y, horizontalalignment, verticalalignment, and loc with the corresponding Matplotlib behavior. Verify that each argument works for both axis-label helpers and add or run focused tests demonstrating the expected placement and alignment.

Written by the indexing model from the issue text.