Add an annotation that indicates a parameter must be used

We just discovered a bug in a recent change caused by forgetting to use a parameter: https://github.com/google/closure-compiler/pull/2740#pullrequestreview-83026869

It would be great if we could annotate the `value` parameter as `@MustBeUsed` on the abstract `apply` method and then ErrorProne would disallow anyone from overriding that method without using `value` somewhere in the body.