relations: support "optional" relation field values

[slint]

It should be possible to have a relation field configured to "optionally" have relation ID, but allow e.g. other attributes. In practice, this means that the following should be a valid record:

class Record(RecordBase):

    relations = RelationsField(
        licenses=PKListRelation(
            'metadata.licenses', 
            record_cls=License, 
            # `True` would be the current behavior
            required_relation=False
        ),
    )

Record.create({
    'metadata': {
        'licenses': [
            # Item with related object filled-in
            {'id': 'mit'},
            # Item without a related object filled-in, but with custom data
            {'title': 'Foobar 3.0', 'description': 'The v3.0 of the Foobar license.'},
        ]
    }
})

Extra

It might be interesting to also support a hybrid of both identifier and custom overrides for the item (e.g. {'id': 'mit', 'description': 'Custom license description'})