Add support for /nodes/{id}/identifiers and /registrations/{id}/identifiers paths

[karenhanson]

An /identifiers/ path is now available through the public OSF V2 API. This should be reflected in the osf-client model. The path lists the DOI and/or ARK ID assigned to the project.

Examples
https://api.osf.io/v2/registrations/cfk8z/identifiers/
https://api.osf.io/v2/nodes/dejq6/identifiers/