Flatpak auth support

Add support for both basic auth and webflow auth. Basic auth is what I'm mostly interested in, but I went ahead and added rudimentary support for the webflow auth as well.

New strings in the basic auth dialog are all taken from flatpak CLI client. If anything there seems wrong we should probably fix them in both places.

Quick demo: https://kalev.fedorapeople.org/gnome-software-basic-auth.webm (the error message at the end indicates that the auth succeeded; I don't have a fully functioning server side here).