Ability to load cross-datastore references in key-value queries

[knstvk]

Currently we cannot load x-ds references using DataManager.loadValues() because a JPQL query can contain only persistent attributes. However, we could achieve the goal if we let users provide an ID of the reference in JPQL and indicate that we actually need the x-ds reference in the properties list. For example:

<keyValueCollection id="keyValueCollection">
    <loader id="keyValueLoader">
        <query>
            <![CDATA[select d.id, d.name, d.db1JpaEntityId from Department d]]>
        </query>
    </loader>
    <properties>
        <property name="id" datatype="uuid"/>
        <property name="name" datatype="string"/>
        <property name="db1JpaEntity" class="com.company.onboarding.entity.Db1JpaEntity"/>
    </properties>
</keyValueCollection>

Here db1JpaEntityId is the persistent attribute that stores ID of the x-ds reference, and db1JpaEntity is the reference itself.