REST API: Sync Past Orders

[n7studios]

Summary

Migrates the Sync Past Orders functionality from using admin-ajax.php to the REST API with a new kit/v1/woocommerce/order/send WP REST API endpoint, similar to the work done on the main Kit Plugin (#953, #954, #959, #1000)

Testing

• RESTAPITest: Added API integration tests against the new REST API endpoint

Checklist

• 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)