HTTPRoutes with invalid ParentRefs are easy to miss

[robscott]

What would you like to be added:
Some kind of warning(s) when a HTTPRoute is attached to an invalid ParentRef. This may involve some combination of:

1. Adding a default status that indicates the unknown status similar to Gateway:

gateway-api/apis/v1/gateway_types.go

Line 44 in 3222422

// +kubebuilder:default={conditions: {{type: "Accepted", status: "Unknown", reason:"Pending", message:"Waiting for controller", lastTransitionTime: "1970-01-01T00:00:00Z"},{type: "Programmed", status: "Unknown", reason:"Pending", message:"Waiting for controller", lastTransitionTime: "1970-01-01T00:00:00Z"}}}

2. Adding some guidance that providers that install/manage CRDs in a cluster can also populate warnings in CRD status in these cases. Although this is not universal, where it is present, it means that we have a single controller that is responsible for CRDs overall, and that could extend to unowned or orphaned CRDs like this.

Why this is needed:
It's unfortunately easy to have an invalid ParentRef in an HTTPRoute without realizing. Because an HTTPRoute with an invalid ParentRef is inherently not tied to a controller, no single controller can populate a warning message when this happens today, leading to further confusion.

[youngnick]

I think that option 1 is fine, we already have the same thing for Gateways and Gateway Listeners.

I think the second is fine as a MAY, but must be an implementation-specific behavior.