introduce FunctionDefinition with a scope to the official collection tree

[RonnyPfannschmidt]

FunctionDefinition was introduced as a internal utility to sort out some bad data issues, however we never made it part of the collection tree

ref #2627 for a practial need example

blocked by #6387

[RonnyPfannschmidt]

blocked by #6387 - need the better from_parent api