Add RegisterElement or equivalent annotation

This idea comes from https://github.com/frankframework/frankframework/issues/2737, where we currently have to use empty setters to allow the XSD from being generated. It would be nice to use annotations on certain elements to dictate which sub-elements are possible on that location.

Though this relation is also present in the [DigesterRules.xml](https://github.com/frankframework/frankframework/blob/bc337acc042fe37dcf199ef9abdf27fc4a24b786/core/src/main/resources/digester-rules.xml#L17C32-L17C33) so I'm unsure why both are needed.