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.

```php
$query = new Query();
$query->criterion = new Criterion\RelationList(
    // Field name
    "relation_field",
    // Operator
    Operator::CONTAINS,
    // Content ID
    42
);
$result = $this->getRepository()->getSearchService()->findContent( $query );
```

Or:

```php
$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 );
```