A USERNAME HTTP authentication mechanism.

This is closely related to FORM authentication but to cover the case where we want to prompt for just a username first and once we know that the subsequent challenge can be customised.

The customisation may be by separate authentication mechanisms then able to offer their own specific FORM variant and perform their own validation.

As an example we may prompt one user for a password whilst prompt a different user for a password and a OTP, we could even use this to decide if we authenticate against a local realm with a credential or redirect via something like KeyCloak.