ListenerSet

        ListenerSet defines a set of additional listeners to attach to an existing Gateway.
This resource provides a mechanism to merge multiple listeners into a single Gateway.

The parent Gateway must explicitly allow ListenerSet attachment through its
AllowedListeners configuration. By default, Gateways do not allow ListenerSet
attachment.

Routes can attach to a ListenerSet by specifying it as a parentRef, and can
optionally target specific listeners using the sectionName field.

Policy Attachment:
- Policies that attach to a ListenerSet apply to all listeners defined in that resource
- Policies do not impact listeners in the parent Gateway
- Different ListenerSets attached to the same Gateway can have different policies
- If an implementation cannot apply a policy to specific listeners, it should reject the policy

ReferenceGrant Semantics:
- ReferenceGrants applied to a Gateway are not inherited by child ListenerSets
- ReferenceGrants applied to a ListenerSet do not grant permission to the parent Gateway's listeners
- A ListenerSet can reference secrets/backends in its own namespace without a ReferenceGrant

Gateway Integration:
  - The parent Gateway's status will include "AttachedListenerSets"
    which is the count of ListenerSets that have successfully attached to a Gateway
    A ListenerSet is successfully attached to a Gateway when all the following conditions are met:
  - The ListenerSet is selected by the Gateway's AllowedListeners field
  - The ListenerSet has a valid ParentRef selecting the Gateway
  - The ListenerSet's status has the condition "Accepted: true"