Modify the "unique" validator to accept additional property names to
narrow the space of rows searched for duplicates.
Example:
Consider `SiteUser` belongsTo `Site` via `siteId` foreign key.
Inside every site, the user email must be unique. It is allowed to
register the same email with multiple sites.
SiteUser.validateUniquenessOf('email', { scopedTo: ['siteId'] });