Flexibilize matching of external user ids and Nextcloud users (SAML, OIDC to LDAP)

Flexibilize matching of external user ids and Nextcloud users (SAML, OIDC to LDAP): The scenario is: When user A logs in via SSO/SAML the external identity provider submits a user id. An LDAP server is often connected to Nextcloud and is backing the logged in user. So when the login happens we have to exactly match the external uid with the Nextcloud internal user id. When a UUID is being used, there is typically no problem. It is not unlikely though that in some setups different values are being used (like last names, email addresses) or UUIDs change due to...