Denotes a field where the parent object will be set. Parent is defined as follows:
- For child property (field), parent is the object
- For a list element, parent is the object that contains the list
- For a map value, parent is the object that contains the map
- For an extension, parent is the object that contains the extension map
If there is no parent found (i.e. the object is used independently), will be set to
null.