LTE attach status and config support
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: ** This MR is rebased on top of the mmcli key/value output changes, !51 (merged).