Pydantic models are now used for Ad-Hoc commands callback data. The `AdHocCommand` model
can (de)serialise the callback by using its import string.
This change has several benefits:
- better data validation.
- the serialisation mechanism will make communication with frontends easier.
- open the door for automatic documentation.
rel 465
the plugin was using python-dbus which is deprecated in the backend in the favor of
TxDBus. Methods calls have been updated, and the plugin works again, but there seems to be
still some issues (warnings in the logs). Those will be fixed later.