Add prompt=create support to send users directly to the account creation page (SAML and OIDC)

[gregjhogan]

Service providers can now send users directly to the Login.gov account creation page instead of the sign-in page by including prompt=create on the authorization request. After the account is created, the user is returned to the service provider exactly as a normal sign-in would.

This uses the OIDC standard Initiating User Registration via OpenID Connect 1.0 prompt=create. The same prompt=create query parameter is honored for SAML as well, so both protocols share one consistent trigger.

Changes:

• SAML and OIDC: unauthenticated requests with prompt=create redirect to the account creation page (only when an SP request is present in the session; behavior is otherwise unchanged).
• OIDC: create added to the accepted prompt values, and advertised via prompt_values_supported in the discovery metadata.
• Tests: unit coverage for both controllers and the OIDC form/discovery metadata, plus IAL1 (full create-account → return-to-SP round trip) and IAL2 (direct landing) feature specs.