Expose SSO identities via Admin API

Via the Admin Users API, the SSO state of a user is visible via the traits array. Sadly, the trait sso:<identity> references the postgres user_sso_identities ID, not the ID from OIDC sub. the sso_identitiy mapping to the SSO ID attribute does not seem to be exposed via (Admin) API yet, which makes figuring out which SSO user a fluxer user corresponds to not possible. This is needed for example for cleanup scripts, so that deleted SSO users can be deleted in Fluxer as well.

Either:

• expose the SSO ID attribute (sub) in the traits as well
• provide an API endpoint to be able to query sso_identity IDs in order to get to the SSO attributes