[review] LTE attach config and status

[Aleksander Morgado]

Hey hey,

See the following MR:
  https://gitlab.freedesktop.org/mobile-broadband/ModemManager/merge_requests/52

The changes will update the 3GPP interface with:
 * A new read-only property "InitialEpsBearer", which points to a
Bearer object that will be automatically created and exposed in DBus
when registered in LTE. This bearer object will define the
settings/status of the initial LTE default bearer in effect at any
given time during runtime.
 * A new read-only property "InitialEpsBearerProperties", a dictionary
which contains the user-defined settings for the initial LTE default
bearer. These settings MAY NOT be the ones effectively used at the end
(e.g. if the network decides to use others, or if none provided by the
user).
* A new method "SetInitialEpsBearerProperties", which allows users to
update the InitialEpsBearerProperties settings.

The changes also include a new "bearer type" enumeration. The normal
user-created bearers will be "default" bearers, while the initial
automatically created bearer will be a "default initial" bearer. We
also define an enum for "dedicated" bearers but we don't have APIs to
support those yet .

The logic is first implemented for MBIM devices, following the
commands defined by Microsoft in the Basic Connect Extensions service:
  https://docs.microsoft.com/en-us/windows-hardware/drivers/network/mb-lte-attach-operations
The ModemManager integration only changes the "home" network settings,
we leave the "partner" and "non-partner" specific settings defined in
the MBIM operations unchanged.

NOTE! The commits are implemented right now on top of the mmcli
key/value output changes, I pushed a MR which also contains those
commits, just to avoid unnecessary rebase work.

Comments welcome!

-- 
Aleksander
https://aleksander.es