webauthn / passkey support. fixes #188

[BotoX]

Properly initialize the extended UsernamePasswordForm with super() and move away from singleton pattern.

This way passkeys work for me.

[arnou]

Hello, thank you for this feature. Would it be possible to document how to implement it in the README? (That would also help me with the PR validation 😉)

[BotoX]

Hi, you're welcome! We (University of Graz) plan on rolling out passkeys very soon for all of our users.
And have already been using your plugin to allow signing in with a 3rd party email address for external users that require a minimal account on our keycloak.

This pull request just makes this plugin behave the same (in regard to passkeys) as the original Username Password Form Authenticator that ships with Keycloak.
Basically follow the official keycloak docs to get passkeys working:
https://www.keycloak.org/2025/09/passkeys-support-26-4
And it shouldn't matter if you use the Keycloak Username Password Form Authenticator or this Attribute authenticator.
You will have passkey/webauthn support in the browser login flow.

Do you want me to elaborate further / add anything to the README?

Or well here, basically I created a brand new realm and then Enabled Passkeys in
Authentication -> Policies -> WebAuthn Passwordless Policy

And voila you have a Sign in with Passkey button:

If you want to use this plugin, make a copy of the browser flow and substitute the regular Username Password Form Authenticator in the copied flow: