The prompt option was being discarded for OAuth authorize URLs after
adding support for the NEXT_PRIVATE_OIDC_PROMPT env var. This meant
select_account (used elsewhere) was not being passed through.
Now defaults prompt to the provided option (or 'login'), and only
overwrites it when a valid OIDC prompt env var is set. Also adds a
type guard to validate the env var value.
Use the `select_account` prompt for SSO OIDC to avoid constantly asking
for credentials to be entered with a client has an existing session with
the SSO provider.
David Nguyen
Lucas Smith
Karlo