TYP: Add type hints for the 'registration' parameter in pygmt.datasets.load_* functions

[seisman]

Description of proposed changes

Address #2812.

Preview: https://pygmt-dev--2867.org.readthedocs.build/en/2867/api/index.html#datasets

Changing registration=None to either

registration: Literal["gridline", "pixel", None] = None,

or

registration: Literal["gridline", "pixel"] = "gridline",

For datasets like earth_age, earth_geoid and earth_mask, the gridline-registrated version exists for all resolutions, thus the default registration should always be gridline, rather than None.