fix: add fields re-ordering for JSON payload

[Soare-Robert-Daniel]

Summary

Because the group field saving payload was switched to JSON in e30abe0, the JSON object does not keep the initial order of its key. Thus, we need to reorder the fields before processing them.

Will affect visual aspect of the product

NO

Test instructions

• Create a new group field with 2 or more input fields
• Check if you can re-order them.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes #404

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