add persistent-keepalive parameter to wireguard peers

[jrester]

NetworkManager supports the persistent-keepalive parameter for wireguard peers. This MR adds this parameter for wireguard peers, so that one can read and modify the persistent-keepalive parameter.

[igo95862]

Hello @jrester

Did you manually add that field or did you use generator in tools/generate-settings-dataclasses-jinja.py? There is an automated script that will generate all settings dataclasses from NetworkManager metadata.