AIP-124: Only one canonical parent

[noahdietz]

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)

[noahdietz]

This guidance is not completely clear and may be difficult to actually identify and enforce. It has continued to be a source of confusion. This should likely be pushed to the backlog until we can fix the guidance language (or remove it).

one for ensuring Standard List only has one parent-style

This is basically already enforced by https://linter.aip.dev/132/request-unknown-fields