Provide mechanism for adding annotations to generated code

In the spirit of the core's [`@AnnotateWith`](https://mapstruct.org/documentation/dev/reference/html/#adding-annotations).

This would solve #3 as well.

Start with the core's @AnnotateWith documentation linked in the issue and review issue #3 for the related requirement. Then identify how this project generates code and decide what annotation mechanism is needed; done means generated code supports the requested annotations and the behavior is covered by appropriate tests.

Written by the indexing model from the issue text.