Previously the requirement that properties ending in `_ref(s)` were
instances of an appropriate type capable of containing a STIX
`identifier` (the current interpretation of section 3.1 of the STIX 2.1
spec) was only enforced for custom observables. This change refactors
the property checks for custom objects to enforce this and the STIX 2.1
property name requirement (also from section 3.1) in a common helper,
therefore extending the enforcement of both requirements to all custom
object types created by downstream projects.
There is a special case carved out for STIX 2.0 observables which are
required to contain "object references" rather than identifiers. This
special logic is encoded in the reference property validation helper and
enabled by a kwarg which is passed exclusively by the custom observable
registration helper.
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