advice/hooks for other login helper APIs to change login status

[npdoty]

Give conditions and/or an API for any other API (fedcm, credential management, webauthn/passkey, maybe even Basic HTTP auth) that a browser might use to help the user log in to change Login Status.

In particular, needs:

• confirmation from the site that the user did successfully log in,
• username/identifier, and
• a way/link for the user to log out.

And then either a method to call, or just a reference in the spec to how login status should be changed.

(Prompted by #44, but applies beyond fedcm.)