Add Identifiers to Entity

[ahwagner]

From #533:

Sequence Reference appears to support only refget accession numbers. How would other types of accession numbers be represented? (as a CURIE in .id?) Is it possible to specify more than one accession number?

@larrybabb suggests this is an opportunity to add a FHIR-like Identifier class and Identifiers property to Entity. Target for 2.1 release unless blocking for an implementation.

Originally posted by @ahwagner in #533 (reply in thread)