Validate the type of properties in primaryIdentifier

[wbingli]

Only String or Number type property can be primaryIdentifier. We should enforce it during schema validation.

[PatMyron]

@wbingli related: see #87 for request for array type primaryIdentifiers

[benkehoe]

Should list-valued identifiers be allowed?

We're creating a resource provider for the sorely-needed S3 bucket notification resource. While the notifications have individual IDs, there is a constraint that no two notifications can overlap in prefix+events. So we had this idea of using that combination as an identifier.

This case is less of a list-valued identifier, and more of a set-valued constraint, but I wanted to raise the question for discussion if others had ideas about when a list-valued identifier would be useful. Given that the primary identifier itself can be list-valued, it seems plausible that sometimes the list wouldn't come from a series of scalar properties but from a single property containing that list.