Implemented EZP-21640: Implement RelationList search criterion

https://jira.ez.no/browse/EZP-21640

Currently, it is not possible to search for Content related (through RelationList) to a specific Content.
In Legacy, this criterion would basically add an additional join to table ezcontentobject_link.

    $query = new Query();
    $query->criterion = new Criterion\RelationList(
        // Field name
        "relation_field",
        // Operator
        Operator::EQ,
        // Content ID
        42
    );
    $result = $this->getRepository()->getSearchService()->findContent( $query );

Or:

    $query = new Query();
    $query->criterion = new Criterion\RelationList(
        // Field name
        "relation_field",
        // Operator
        Operator::IN,
        // Content ID
        array( 42, 43, 44 )
    );
    $result = $this->getRepository()->getSearchService()->findContent( $query );