broadband-modem: enable and handle +CGEV indications

The +CGEV indications allow us to get notified of packet domain
events like network-initiated or ME-initiated disconnections on
specific connected contexts, as well as full PS detach events (all
contexts disconnected). If the modem supports these indications, we
will enable them with +CGEREP and will then process them when they are
emitted by the modem.

If a full PS detach event happens, we will explicitly disconnect all
connected bearers.

If a deactivation inidication is received for a single context, we
will look for the associated bearer by CID and explicitly disconnect
it.