Add Gateway to core reference from kinds

Add Gateway to the list of core kinds implementations must support in
ReferenceGrantFrom. Gateway Listener core support requires
implementations support references to TLS Secrets in other namespaces,
and these references require permission from a ReferenceGrant in the
target namespace.

apis/v1alpha2/referencegrant_types.go

@@ -92,13 +92,14 @@ type ReferenceGrantFrom struct {
 	Group Group `json:"group"`
 
 	// Kind is the kind of the referent. Although implementations may support
-	// additional resources, the following Route types are part of the "Core"
+	// additional resources, the following types are part of the "Core"
 	// support level for this field:
 	//
 	// * HTTPRoute
 	// * TCPRoute
 	// * TLSRoute
 	// * UDPRoute
+	// * Gateway
 	Kind Kind `json:"kind"`
 
 	// Namespace is the namespace of the referent.