Allows users define customized key type annotations for map bindings by annotating an annotation
 * of a {@code Map}'s key type. The custom key annotation can be applied to methods also annotated
 * with {@literal @}{@link ProvidesIntoMap}.
Allows users define customized key type annotations for map bindings by annotating an annotation
 * of a {@code Map}'s key type. The custom key annotation can be applied to methods also annotated
 * with {@literal @}{@link ProvidesIntoMap}. | MapKey.java