HHH-2907 Custom generator annotation types

@sebersole: Hi Steve, this PR provides support for custom generator annotation types. Based on the generic mechanism, there is the following annotation types provided:

* `@CreationTimestamp`
* `@UpdateTimestamp`
* `@GeneratorType`, allowing to specify explicitly a `@ValueGenerator` type
* `@Generated` has been retrofitted into a generator annotion type based on this mechanism