Add load_moon_relief to load Moon relief dataset in various resolutions and registrations

[seisman]

Description of proposed changes

Preview: https://pygmt-dev--2674.org.readthedocs.build/en/2674/api/generated/pygmt.datasets.load_moon_relief.html

import pygmt
from pygmt.datasets import load_moon_relief
grid = load_moon_relief(resolution="15m")

fig = pygmt.Figure()
fig.grdimage(grid, cmap="@moon_relief.cpt")
fig.show()

Address #2674.

Reminders

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[seisman]

Please note that extra_attributes is empty here because I have no idea about the horizontal_datum attribute for the dataset.