RFC: Add desktop portal diagnostic interface

[tytan652]

I thought about how to improve Door Knocker like:

• being able to fetch if the access portal is present which if missing cause some portal to be disabled
• avoid relying on --talk-name=org.freedesktop.Flatpak and flatpak-spawn but add a --talk-name to a specific interface
• potentially get backends in use and the portal version implemented

At the same time I tried to make sure that this interface is not meant only for my application but also could be used by desktop environment to have a good insight of the state of the portal service.

This interface is not meant to be accessible directly by Flatpak application.