Instead of using a predefined set of string values for 'ip-type' in
Modem.CreateBearer() and Simple.Connect(), we'll use an enumeration. The
implementation will then need to convert the requested IP family type to e.g.
the correct PDP type in 3GPP modems.
This change also consolidates the use of enums in dictionary properties when
possible to do so, as with the Rm Protocol.
Actions on modems from specific interfaces, like 3GPP, will also need the modem
to be specified.
The new 'Common' option group will handle all options to specify objects.
