feat: implement prompt=create: Initiating User Registration via OpenID Connect

[nynymike]

Initiating User Registration via OpenID Connect 1.0 - draft 06

Several years of deployment and implementation experience with OpenID Connect Core 1.0 [OpenID.Core] has uncovered a need, in some circumstances, for the client to explicitly signal to the OpenID Provider that the user desires to create a new account rather than authenticate an existing identity.

This specification allows the client to indicate to the OpenID Provider that the user desires to create an account improving the user experience by reducing the friction for the user of finding the sign-up link on the default login page.

This specification defines a new value for the prompt parameter.