Allow saving extra columns to junction table

[sspat]

Currently it the behavior code it is hard coded that the yii\db\BaseActiveRecord->link method is called without a possible third parameter.

The third parameter is used for passing additional columns data to be saved to the junction table in a many-to-many relationship through a junction table.
http://www.yiiframework.com/doc-2.0/yii-db-baseactiverecord.html#link()-detail

It would be nice if the behavior would allow to pass and save/update such additional columns in the junction table utilizing this native Yii2 method.

For example it could be made with declaring a callback in the behavior configuration that returns an array of the junction table column values for each ActiveRecord instance being saved,

An example:

/** @inheritdoc */
    public function behaviors()
    {
        return [
            'saveRelations' => [
                'class'     => SaveRelationsBehavior::class,
                'relations' => [
                    'photos'
                ],
                'junctionTableProperties' => [
                    'photos' => function (\yii\db\ActiveRecord $record) :array {
                        return [
                            'order' => $record->order
                        ];
                    }
                ]
            ]
        ];
    }