Select the client-registration approach for an authorization server,
per the spec priority order ("Client Registration Approaches"):
pre-registration, then Client ID Metadata Documents (SEP-991), then
Dynamic Client Registration, then prompting the user. havePreRegistered
indicates the caller already holds a client_id for this AS.
Select the client-registration approach for an authorization server, per the spec priority order ("Client Registration Approaches"): pre-registration, then Client ID Metadata Documents (SEP-991), then Dynamic Client Registration, then prompting the user. havePreRegistered indicates the caller already holds a client_id for this AS.