Automatic merge from submit-queue (batch tested with PRs 54761, 54748, 53991, 54485, 46951). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
admission: unify plugin constructors
It's common in Go to return the actual object in constructors, not **one interface**
it implements. This allows us to implement multiple interfaces, but only have
one constructor. As having private types in constructors, we export all plugin structs, of course with private fields.
Note: super interfaces do not work if there are overlapping methods.
