Change usage of AT_CellularContext::stack_type_supported to
AT_CellularBase::get_property. This way we can rid of
targets overriding stack_type_supported and delete
unnecessary classes and simplify new targets.
After AT_CellularNetwork::has_registration was replaced with
CellularProperties and better
AT_CellularNetwork::set_access_technology_impl default
implementation we can delete most of the target specific classes
that inherit AT_CellularNetwork.
CellularDevice class own new state machine. Now we don't have to expose state machine
which may change and so we don't have to make API changes if that happens.
EasyCellularConnection uses now CellularDevice instead of old state machine.