Remote datasets: Add "load_earth_mean_sea_surface" to load "CNES Earth Mean Sea Surface" dataset

[yvonnefroehlich]

Description of proposed changes

Add load_earth_mean_sea_surface to download the "CNES Earth Mean Sea Surface" dataset and load into a xarray.DataArray:

• Official documentation: https://www.generic-mapping-tools.org/remote-datasets/earth-mss.html
• Recipe: https://github.com/GenericMappingTools/gmtserver-admin/blob/master/recipes/earth_mss.recipe

Adresses #2431

Preview: https://pygmt-dev--3717.org.readthedocs.build/en/3717/api/generated/pygmt.datasets.load_earth_mean_sea_surface.html

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