Support WebAuthn hints

[iinuwa]

WebAuthn hints are ways for websites to express what they believe to be the most likely way that the user will authenticate with their credential. They may know, for example, that a user's only WebAuthn credential is a security key, so they'll want the platform to display that to make it easier for them to use.

WebAuthn supplies multiple ways to convey this information, including transports and authenticatorAttachment, with hints being the latest of the methods. Let's start with hints for now. If a lot of sites use the other methods and it makes it harder for users, we can consider adding those later.

[AlfioEmanueleFresta]

Completely agree we should support this.

I can think of some scenarios where only RPs can only hint at transports per credential - I'd like to support these.

@iinuwa WDYT of having libwebauthn parse all 3 hint locations as per spec, and merge the result in a unified, sorted field for credentialsd to consume?

[iinuwa]

That sounds like a good plan!

This will require some more UI work too. Right now, we're using a StackPage to switch between the different views, but there's some weirdness. Essentially, we'd want to reference the same hybrid QR data, USB PIN entry field, etc. from multiple views to give visual priority to different transports, but keeping all the signal handling the same. But you can only bind a GTK property to a single GTK value at a time. I'm not a GTK wizard, but I think this means that we need another layer of indirection to make this work.