Add support for D-Bus virtual devices

[ueno]

This enables access to authenticator devices through the D-Bus based protocol rather than low-level communication protocols such as USB HID. The main motivation of this is to allow isolated clients (e.g., Firefox Flatpak) to run without unnecessary privileges on the host.

The D-Bus server implementation is mostly done by our colleague Norbert as part of his internship at Red Hat. The protocol is still not fixed, so use my branch if you are interested in testing it.