FieldRelation Search Criterion
searches for content based on the Content items it has Relations to.
target- string representing the identifier of the Field containing Relations
operator- Operator constant (IN, EQ, GT, GTE, LT, LTE, BETWEEN)
value- array of ints representing the Relation content IDs to search for
Use of IN means the Relation needs to have one of the provided IDs, while CONTAINS implies it needs to have all provided IDs.
$query->query = new Criterion\FieldRelation('relations', Criterion\Operator::CONTAINS, [55, 63]);