Structure Generation Hook for DataMarkers

I want to be able to modify how already existing structures handle datamarkers saved in structure files.

Example of this are shulker generation in end cities or evoker/vindicator generation in mansions.

My suggestions would be an event hook being called before the vanilla logic, allowing mods to change the behaviour or add mob spawns on structure generation.

This event could also be canceled to prevent the vanilla logic to be called if wanted.

I have created a pull request implementing the feature