Add Parent Item to order item in repository

[Jeroenll]

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)

[ihor-sviziev]

Hi @JeroenVanLeusden,
Your changes looks good to me. Could you cover them with tests?

[Jeroenll]

Sure, hopefully have some time by end of this week. Will ping you when ready.

Edit:
@ihor-sviziev Struggling to make up some good test cases any ideas to help me setting up?

[ihor-sviziev]

@JeroenVanLeusden I think good option there will be adding new integration test that will check after getting parent item is present

[Jeroenll]

Have time by end of the week to update the PR, just so you know ;)

[Jeroenll]

@ihor-sviziev PR is updated with test.

[magento-engcom-team]

Hi @ihor-sviziev, thank you for the review.
ENGCOM-1531 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 2.2.5 release.