Use generics for waitFor (delaying workqueue) in client-go

With this change, we can typically avoid an extra heap allocation when
calling AddAfter with a positive duration (which causes the creation of
a waitFor object). This is because workqueues are typically used with
string keys, and casting a string (or more generally, non-pointer types)
to an `interface{}` will cause an heap escape / allocation.

Ater this change, there is no longer any usage of `type t interface{}`,
which was creating some confusion after the switch to generics in the
workqueue package.

Co-authored-by: Quan Tian <quan.tian@broadcom.com>
Signed-off-by: Antonin Bas <antonin.bas@broadcom.com>

Kubernetes-commit: 53ddffb55709857fec6bddbd5ca4ca1b03f7da97

util/workqueue/rate_limiting_queue_test.go

@@ -32,7 +32,7 @@ func TestRateLimitingQueue(t *testing.T) {
 		clock:           fakeClock,
 		heartbeat:       fakeClock.NewTicker(maxWait),
 		stopCh:          make(chan struct{}),
-		waitingForAddCh: make(chan *waitFor, 1000),
+		waitingForAddCh: make(chan *waitFor[any], 1000),
 		metrics:         newRetryMetrics("", nil),
 	}
 	queue.TypedDelayingInterface = delayingQueue