C API: add proj_get_domain_count(), proj_get_area_of_use_ex() and proj_get_scope_ex()

[rouault]

Up to now, the C API only handled one single domain/usage par object, but there are objets in the database that have several ones.

refs #3246

[kbevers]

Looks alright to me. I'm not sure how the _ex should be understood. Extended? Also, remember to update docs/source/development/reference/functions.rst before merging.

[rouault]

yes, _ex is for extended. We have a proj_create_vertical_crs_ex() in proj_experimental.h

There's no need to explictly extend the documentation, as those functions below to the iso19111_functions Doxygen group that is ingested through Breathe, with the ".. doxygengroup:: iso19111_functions" directive at end of docs/source/development/reference/functions.rst

[rouault]

Regarding automatic documentation, cf https://osgeo-proj--3588.org.readthedocs.build/en/3588/development/reference/functions.html#c.proj_get_domain_count

[kbevers]

There's no need to explictly extend the documentation, a

Right, I forgot about that. Everything's good then.