AIP-124: Only one canonical parent

AIP-124 states:
> A resource **must** have at most one canonical parent, and List requests **must not** require two distinct "parents" to work.

This might be two rules:
- one for ensuring that a resource does not have another field that would act as a parent (perhaps a field that has a resource reference to a resource that is actually a parent of this one - seems hard/not that effective)
- one for ensuring Standard List only has one parent-style resource reference (perhaps only one resource reference per-standard list request)