https://docs.oasis-open.org/security/saml/v2.0/saml-core-2.0-os.pdf (Persistent Identifier 8.3.7, Transient Identifier 8.3.8)
https://wiki.shibboleth.net/confluence/display/CONCEPT/NameIdentifiers
https://wiki.geant.org/display/eduGAIN/Identifier+Attributes
https://wiki.refeds.org/display/STAN/eduPerson+2020-01#eduPerson2020-01-eduPersonTargetedID
https://kantarainitiative.github.io/SAMLprofiles/saml2int.html#_subject_identifiers (3.1.3, SP)
https://kantarainitiative.github.io/SAMLprofiles/saml2int.html#_subject_identifiers_2 (4.1.3, IdP)