Rename ObjectFieldStorage to ObjectStructure and move it to a more appropriate package

It's currently in a "dsl" package, unlike similar enums such as org.hibernate.search.engine.backend.types.Projectable. Maybe move it to that package? Or to org.hibernate.search.engine.backend.document.model?

This may also be an opportunity to rename this concept to ObjectStructure, which is arguably more fitting.
If we did, we could just deprecate the old enum and corresponding annotation attributes/DSL methods in the mapping.