Add support for GSM MMS APNs

It would be nice if NetworkManager have a way to store mms APNs. It'll help gnome-control-center manager MMS settings (as it now manages Internet APNS), which can be used by tools like mmsd.

the following settings shall be required for NMSettingGsm:

"apn-type" : A comma separated value of at least "default" and "mms" (and possibly other values), defaults to "default" if not set.
"mmsc"
"mms-proxy"
"mms-port"

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