GEP-957: Add `RouteConditionReason` which could indicate a route not matching parentRefs Ports

[pmalek]

What would you like to be added:

https://gateway-api.sigs.k8s.io/geps/gep-957/ introduce a concept of port matching, so that a route can specify parentRef.port.

Unfortunately, an appropriate reason which could be used for Accepted condition does not exist among the predefined list of reasons:

gateway-api/apis/v1beta1/shared_types.go

Lines 189 to 223 in 2f7ab68

	const (
	// This condition indicates whether the route has been accepted or rejected
	// by a Gateway, and why.
	//
	// Possible reasons for this condition to be true are:
	//
	// * "Accepted"
	//
	// Possible reasons for this condition to be False are:
	//
	// * "NotAllowedByListeners"
	// * "NoMatchingListenerHostname"
	// * "UnsupportedValue"
	//
	// Possible reasons for this condition to be Unknown are:
	//
	// * "Pending"
	//
	// Controllers may raise this condition with other reasons,
	// but should prefer to use the reasons listed above to improve
	// interoperability.
	RouteConditionAccepted RouteConditionType = "Accepted"
	// This reason is used with the "Accepted" condition when the Route has been
	// accepted by the Gateway.
	RouteReasonAccepted RouteConditionReason = "Accepted"
	// This reason is used with the "Accepted" condition when the route has not
	// been accepted by a Gateway because the Gateway has no Listener whose
	// allowedRoutes criteria permit the route
	RouteReasonNotAllowedByListeners RouteConditionReason = "NotAllowedByListeners"
	// This reason is used with the "Accepted" condition when the Gateway has no
	// compatible Listeners whose Hostname matches the route
	RouteReasonNoMatchingListenerHostname RouteConditionReason = "NoMatchingListenerHostname"

This probably should also be mentioned in the GEP itself.

Why this is needed:

To indicate that a route couldn't be matched with a parent because a matching port wasn't found.

Proposal:

Introduce something along the lines (similar to RouteReasonNoMatchingListenerHostname):

const (
	// This reason is used with the "Accepted" condition when the Gateway has no
	// compatible Listeners whose Port matches the route
	RouteReasonNoMatchingListenerPort RouteConditionReason = "NoMatchingListenerPort"
)

#1511 (comment)

[shaneutt]

We talked about this at the community, please feel free to add this but let's make sure that we get conformance tests 👍

Also, if we can do this very quickly we're in favor of adding this to the soon-to-release v0.6.0, but it's going to have to be pretty quick. Otherwise we'll aim for v0.7.0.

[robscott]

Agree with @shaneutt, this is a reasonable addition. We don't need a GEP for just adding a new reason like this, but accompanying with a conformance test would be good.

One tiny nit, I think I'd recommend something more generic, such as "NoMatchingParent" since it could also be restricted by SectionName.

[pmalek]

Created #1516, taking your suggestion into account @robscott. Let me know if there's more test cases you'd like to cover in that PR.