[attribute form] Add parent feature scope when adding/editing a child featur through the relation editor widget

[nirvn]

Description

This PR provides a useful current_parent_* scope when adding and editing child features through the attribute form's relation editor widget.

For feature addition, it comes in extremely handy when the parent feature is also being added. In this case, the parent feature itself can't be retrieved (since it hasn't even been added to the edit buffer), nor can its attributes or geometry.

Fixes #48915.

[uclaros]

Does this fix #48915 then? (... and also properly fix #40194 ?)

[nirvn]

@uclaros , it does implement what's requested in #48915 yes :)

[github-actions]

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