[WIP] PIM-2672 : add an optional related-entity parameter in view tree requests

| Q                    | A
| -------------------- | ---
| Bug fix?             | N
| New feature?         | Y
| BC breaks?           | N
| CI currently passes? | ?
| Tests pass?          | Y
| Scenarios pass?      | ?
| Checkstyle issues?   | N
| Changelog updated?   | N
| Fixed tickets        | PIM-2672 (partially)
| Doc PR               |