Obtain corner vertices of Regions

A new method returning the corner vertices of regions, for instance the four corners of a RectangleSkyRegion, as an array of SkyCoord, would be useful.

Use case: to define a MoC of the region using [MOC.from_polygon](https://cds-astro.github.io/mocpy/examples/examples.html#create-a-moc-from-a-concave-polygon)

The issue names no implementation files or tests. Start by locating the region classes and examining how RectangleSkyRegion represents its geometry and coordinates. Done means a consistent method returns corner SkyCoord arrays that can be passed to MOC.from_polygon, with appropriate coverage for the relevant region types.

Written by the indexing model from the issue text.