public identifiers, entities interfaces

[karenetheridge]

As in JSON::Schema::Modern, make a read-only identifiers interface available on the OpenAPI::Modern object. see karenetheridge/JSON-Schema-Modern#88

It would also be useful to identify all the "entities" known to the OpenAPI description, e.g. $openapi->entities_list('path-item') would list the canonical locations of all the path-items.