rdp: Add methods for getting and setting system credentials

Control Center needs to be able to find out the username and password for the RDP server, along with setting up TLS certificates, and also enabling and disabling the server, all without having to run grdctl (which requires screen scraping output and causes unavoidable polkit dialogs at inopportune moments)

This merge request adds methods for getting and setting credentials, importing certificates, and enabling and disabling RDP.

(For the system service these features are only available to users that have permission to perform the

org.gnome.remotedesktop.configure-system-daemon

polkit action)