Report error for policy when ancestor status slice is full

[kate-osborn]

Status on policies contains a slice of Ancestors, which has a max size of 16. If the slice is full, we consider the policy unimplementable but we do not signal this to the user in any way. We effectively ignore the policy.

The spec says

If this slice is full, implementations MUST NOT add further entries.
Instead they MUST consider the policy unimplementable and signal that
on any related resources such as the ancestor that would be referenced
here. For example, if this list was full on BackendTLSPolicy, no
additional Gateways would be able to reference the Service targeted by
the BackendTLSPolicy.

We need to figure out a way to signal this on the ancestor resource. One way to do this would be to add a condition to the ancestor. But this may overflow the ancestor's conditions.

[kate-osborn]

See this thread for more info: #1940 (comment)