[Forwardport] Customer group extension attributes not carried over on save

[mageprince]

Original Pull Request

#16254

Description

When saving a customer group, extension attributes should be copied to the new data model that is returned. Unfortunately, this action was not completed and results in unexpected behavior.

Fixed Issues (if relevant)

Manual testing scenarios

1. Ensure that saving a customer group does not throw any errors.
2. Ensure that utilizing extension attributes for a customer group now carries over through the save process.

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)

[magento-engcom-team]

Hi @mageprince. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
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[magento-engcom-team]

Hi @orlangur, thank you for the review.
ENGCOM-2269 has been created to process this Pull Request

[magento-engcom-team]

Hi @mageprince. Thank you for your contribution.
We will aim to release these changes as part of 2.3.0.
Please check the release notes for final confirmation.