Allow `ChildToParent` & `ParentToChild` enum aliases for `TransformRelation`

[Wumpf]

• Followup to Introduce new enum component to express from_parent #6863

At least in Python we should also offer ChildToParent and ParentToChild as valid variants since this is what a lot of people have in their code. We would however map that to above TransformRelation.

The ideal solution for this would be to allow aliases on our enum types.