refactor: wwan.json schema fic emun > enum
add: backend method to rollback (get the proper wan iface)

Fetching of SIM card status could get stuck in certain conditions, which
will in turn block foris-controller.

For example:
Querying SIM card status through its control device (/dev/cdc-wdmX)
could get stuck for the first invocation, but will work fine on second
and any subsequent run.

Apply time limit on `uqmi` command run, so we won't get stuck and can at
least report that fetching SIM status failed this time.

This implement initial functionality to use QMI modem(s) as wireless WAN.

Only one modem at the time acting as WAN is currently supported.
The expected setup is just one modem - i.e. Turris router with LTE kit -
but code can handle multiple attached modems.

In that scenario all detected modems are offered as a choice, but there
can be only one selected and designated as WAN.

Following API functions are currently implemented:

* get_settings
* update_settings