Allow multiple named subqueries for @MappingSubquery

[beikov]

Right now, the SubqueryProvider only allows adding one subquery, but it would be awesome if we allowed multiple named subqueries.
Maybe we can add a sub-interface for this purpose?

Possible workaround is to use a SubqueryProvider with fromValues e.g.:

public class NameAndOrgCountCorrelationProvider implements SubqueryProvider {

        @Override
        public <T> T createSubquery(SubqueryInitiator<T> subqueryBuilder) {
            return subqueryBuilder.fromValues(Integer.class, "i", List.of(1))
                .selectSubqueries("q1 + q2")
                    .with("q1")
                        .from(Name.class, "n")
                        .where("n.relationship.id").eqExpression("EMBEDDING_VIEW(relationship.id)")
                        .bind("nr").select("count(*)")
                    .end()
                    .with("q2")
                        .from(Org.class, "o")
                        .where("org.relId").eqExpression("EMBEDDING_VIEW(relationship.id)")
                        .bind("nr").select("count(*)")
                    .end()
                .end()
            .end();
        }
}