refactor: Add backend abstration to firewall_lib

Introduce a backend class into firewall_lib, and move all `FirewallClient`
operations into it. This gets rid of some global variables, separates the
Ansible/parameter parsing from the firewalld operations, shrinks the massive
main() function, and allows us to add another implementation for offline mode
in the future (based on `firewalld-offline-cmd` or XML editing).

This does not change the logic or code much, other than adding a
truckload of `self.` prefixes and some `black` reformatting. There is
still way too much global state, which makes the code hard to
understand, but let's clean this up in steps.

The only other changes aside from moving are the elimination of
`handle_interface_permanent()` and `set_the_default_zone()` global functions.
They are FirewallClient API specific, only get called once (the former with two
unused parameters), and are simple enough to just get inlined.

Author: martinpitt