Clarify support for Gateway API for Mesh

[LiorLieberman]

flagger/pkg/apis/flagger/v1beta1/canary.go

Lines 163 to 166 in 27daa2c

	// Gateways that the HTTPRoute needs to attach itself to.
	// Must be specified while using the Gateway API as a provider.
	// +optional
	GatewayRefs []v1beta1.ParentReference `json:"gatewayRefs,omitempty"`

I was looking into the gateway api support for flagger, and came across the support for gateway api. It looks like there is a GatewayRefs field that should specify Gateways that should be the parentRefs of the HTTPRoute.

However, Gateway API evolved to support mesh traffic as well, but having Service as a parentRef for HTTPRoutes.

From a brief look at the code it looks like this will support specifying Service in GatewaysRef.

Would appreciate clarification if:

a. The above is true and it will support it
b. what are the options to make sure we represent/document the support for Services there for Gateway API Mesh usecases? I assume GatewaysRef is not going to changed, but maybe another ServiceRef field plus CEL expressions to disallow putting Service as GatewaysRef and Gateways in ServiceRef (though this CEL may be a breaking change)

Thanks!