v4 API: Save updated tokens on refresh

[n7studios]

Summary

Stores the refreshed Access + Refresh Tokens when an API request is made and the Access Token has expired.

Testing

• testAccessTokenRefreshedAndSavedWhenExpired: Tests that the new Access and Refresh Tokens are saved in the Plugin's settings when the WordPress Libraries make a call to an API endpoint with an expired Access Token.

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)