Update TargetREf in Policy GEP

[candita]

Discussion generated from GEP-1897 points out that TargetRef could use some updates.

1. All policy resources must include TargetRef with the fields specified here:

   gateway-api/apis/v1alpha2/policy_types.go

   Lines 24 to 41 in a33a934

   	type PolicyTargetReference struct {
   	// Group is the group of the target resource.
   	Group Group `json:"group"`
   	// Kind is kind of the target resource.
   	Kind Kind `json:"kind"`
   	// Name is the name of the target resource.
   	Name ObjectName `json:"name"`
   	// Namespace is the namespace of the referent. When unspecified, the local
   	// namespace is inferred. Even when policy targets a resource in a different
   	// namespace, it MUST only apply to traffic originating from the same
   	// namespace as the policy.
   	//
   	// +optional
   	Namespace *Namespace `json:"namespace,omitempty"`
   	}

2. Policy resources may also choose to include SectionName and/or Port in TargetRef following the same mechanics of ParentRef

Then we can provide additional TargetRef types that extend the base TargetRef with the different possible combinations of these fields. That feels like it could be handled in a separate smaller update to the policy GEP.

Originally posted by @robscott in #2113 (comment)

[arkodg]

happy to pick this one up, @robscott does this warrant a discussion first, or can I start with a new GEP or edit the existing GEP-713 ?

[youngnick]

I think that #2128 needs to merge before we make any more Policy changes. After that, yes, this should update GEP-713.

[shaneutt]

I think that #2128 needs to merge before we make any more Policy changes. After that, yes, this should update GEP-713.

+1, let's get #2128 done and then update the existing doc.

[shaneutt]

/triage accepted
/assign @arkodg