Add secondary API key

[sal4sup]

All Submissions:

• Does your code follow WooCommerce and WordPress standards?
• Have you successfully run tests with your changes locally?
• Have you tested both blocks/non-blocks cart/checkout?
• Have you tested the cart and checkout as both a logged-in user and a guest?
• Have you successfully placed an order as both a logged-in user and a guest?
• Did you have the query monitor plugin active during all testing?

Changes proposed in this Pull Request:

Closes # .
https://app.clickup.com/t/868j4b857

How to test the changes in this Pull Request:

1.Go to Postnl settings
2. Validate the new API key
3.

Other information:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.