GEP-1713: ListenerSets - Standard Mechanism to Merge Gateway Listener…

…s (rev 2) (#3213)

* Second Revision of GEP-1713

* fix minor nits

* Drop section highlighting failure cases we can't handle

* add enum validation

* Describe the 'meta' details about this feature

* attaching to the Gateway's sectionName but through the ListenerSet kind is not allowed

* ListenerSet can have multiple parents

* Allow multiple parentRefs and adjust language around PolicyAttachment

* clarify which section has listener ordering

* fix heading levels

* allow no listeners on the Gateway spec

* add back istio use case

* update gateway conditions paragraph

* don't say concatenate

* drop clause saying invalid listeners affects other listeners

* clarify route attachment

* split listenersetconditions into two sections: top-level and listenerstatus

* clarify where the conflicted condition is set

* relabel introduction => use cases

* limit parents to 2 and listeners to 16

* use separate listener and status types

* ListenerSet attachment is opt-in

* fix formatting

* fix more formatting

* include potential goals from the gateway hierarchy document

* fix language that allows a route to attach to both a listener set and a gateway at the same time

* if a route tries to attach to a non-existent listener on the listener set then Accepted should be false

* ListenerStatus => ListenerEntryStatus

* add a blurb about listenerset+gateway listeners are joined as a list

* Gateways that fail should also fail their attached ListenerSets

* don't leak secrets

* update title

* mention dummy listener for the first release

* tweak the name of the AttachedListenerSets condition

* rob's suggestions

* bump listener count

* just re-use GatewayTLSConfig

* use v1 api version for httproute

* replace route with ListenerSet in godoc

* include listener precedence in godoc

* address mlavacca comments

* ListenerSet in now in the x-k8s.io API group

* make port optional

* update section on ListenerEntry changes

* single parentRef for now