Add load_venus_relief to load Venus relief dataset in various resolutions

[seisman]

Preview: https://pygmt-dev--2906.org.readthedocs.build/en/2906/api/generated/pygmt.datasets.load_venus_relief.html#pygmt.datasets.load_venus_relief

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

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

Address #2431.