Add geoboundaries as possible source for build_shapes

[davide-f]

Describe the feature you'd like to see

Geoboundaries is an CC-BY alternative to gadm that is more permissive from a licensing point of view and may be an alternative with respect to availability of servers.
We may add an option in build_shapes to support this data source as an alternative for gadm.

This notebook can offer same code for the interfacing:
https://github.com/davide-f/pypsa-earth/blob/fake_gadm/create_fake_gpkg.ipynb

Note that as we have 3 gadm functions in the workflow, it would be advisable to first drop that duplication