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/queue.go

@@ -212,7 +212,6 @@ type Typed[t comparable] struct {
 }
 
 type empty struct{}
-type t interface{}
 type set[t comparable] map[t]empty
 
 func (s set[t]) has(item t) bool {