ussd: allow defaulting to encoded or unencoded per plugin

Submitted by Aleksander Morgado

Assigned to ModemManager bug user

Link to original bug (#87247)

Description

Originally reported at: https://bugzilla.gnome.org/show_bug.cgi?id=733690

Seems some ZTE modems have problems when we default to using encoded USSD, so defaulting to unencoded in this case could be a sane default.

Version: git master