add relay support

[CzarofAK]

Would it be possible to add also a relay support? The cerbo only offers two, which is no enough for some users.
Maybe in combination with ESPhome.

[freakent]

This driver can only register devices for apis that Victron provides support for. There is no device for simply "relay". Here's the full list:

https://github.com/victronenergy/venus/wiki/dbus