Mobile APN: Support for multiple APNs

Hello!

One feature in Modem Manager 1.18 is the ability to manage multiple APNs:

https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/merge_requests/447

This is desirable in the context of MMS (and possibly other instances), as there are some carriers that seperate an MMS APN and a data APN: https://gitlab.com/kop316/mmsd/-/issues/5

It would make the most sense for the controls to be in g-c-c.

I am filing this to start that discussion on where it should go, how it should work, etc.