Enable device pairing using homeassistant builtins

[gluap]

This was brought up as gluap/pyduofern#28

Currently pyduofern uses an old workaround: to allow pairing, a "service" has to be called. By now Homeassistant would allow for a nice user-friendly gui workflow. Another relic that is likely related is that we still write duofern.json in the background which shouldn't be necessary any more.

This bug is about
-> move pairing to the GUI framework that other integrations use
-> in the process: maybe get rid of duofern.json

Hopefully one outcome would be a list of services and entities as is provided for other integrations:

[bcutter]

Isn't the new pairing flow in 0.5.x releases adressing this?
Did not pair a new shutter yet (but need to in the next days) so not sure about that.