DRA: add DRAControlPlaneController feature gate for "classic DRA"

In the API, the effect of the feature gate is that alpha fields get dropped on
create. They get preserved during updates if already set. The
PodSchedulingContext registration is *not* restricted by the feature gate.
This enables deleting stale PodSchedulingContext objects after disabling
the feature gate.

The scheduler checks the new feature gate before setting up an informer for
PodSchedulingContext objects and when deciding whether it can schedule a
pod. If any claim depends on a control plane controller, the scheduler bails
out, leading to:

    Status:       Pending
    ...
      Warning  FailedScheduling             73s   default-scheduler  0/1 nodes are available: resourceclaim depends on disabled DRAControlPlaneController feature. no new claims to deallocate, preemption: 0/1 nodes are available: 1 Preemption is not helpful for scheduling.

The rest of the changes prepare for testing the new feature separately from
"structured parameters". The goal is to have base "dra" jobs which just enable
and test those, then "classic-dra" jobs which add DRAControlPlaneController.

pkg/scheduler/framework/plugins/feature/feature.go

@@ -20,6 +20,7 @@ package feature
 // This struct allows us to break the dependency of the plugins on
 // the internal k8s features pkg.
 type Features struct {
+	EnableDRAControlPlaneController              bool
 	EnableDynamicResourceAllocation              bool
 	EnableVolumeCapacityPriority                 bool
 	EnableNodeInclusionPolicyInPodTopologySpread bool