Allow aliases in GeoTIFF CRS

[desruisseaux]

In the planetary working group, there is a need for aliases because names or identifiers of some asteroids (for example) can change. The ISO 19111 specification already supports aliases, but not GeoTIFF. A possible approach would be as below:

• Adopt the GDAL/ESRI convention described in GitHub issue 59.
• Add the following amendment: keys can be repeated. If a key is repeated (not necessarily consecutively), all occurrences after the first one are aliases. For example in "GCS Name = my CRS|Datum = My datum|Ellipsoid = my ellipsoid|Datum = My Datum alias|", "My datum" would be the primary datum name and "My datum alias" would be an alias for that datum.

[desruisseaux]

This pull request defines the structure of GeodeticCitationGeoKey. Some requirements in GeoTIFF specification seem to implicitly require a similar structure for VerticalCitationGeoKey and ProjectedCitationGeoKey as well — see https://github.com/opengeospatial/geotiff/issues/59#issuecomment-1761681813. Current pull request does not cover those two latter cases.