[Port 2.3-develop] Add parent item to order item

[Jeroenll]

Port of #14614

Description

When using GET /V1/orders/items/{id} the parent item isn't set. This PR will add the parent item if the parent_item_id is set.

Fixed Issues (if relevant)

1. None

Manual testing scenarios

1. Order a configurable product.
2. Fetch the ordered simple product using GET /V1/orders/items/{id}.
3. parent_item_id is set but parent_item isn't.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[Jeroenll]

@ihor-sviziev Can you also process the port to 2.3-develop?

[magento-engcom-team]

Hi @ihor-sviziev, thank you for the review.
ENGCOM-1532 has been created to process this Pull Request

[magento-engcom-team]

Hi @JeroenVanLeusden. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.3.0 release.