v4 API: Exchange API Key and Secret for Access and Refresh Tokens on Plugin Upgrade

[n7studios]

Summary

Automatically exchanges an existing Plugin installation's API Key and Secret for Access and Refresh Tokens when the Plugin is upgraded to 2.5.0 or higher, which introduces using the v4 API with OAuth.

Testing

• testExchangeAPIKeyAndSecretForAccessToken: Tests that an existing Plugin installation's API Key and Secret are exchanged for Access and Refresh Tokens, which are then tested as working by making API calls through loading the Plugin's settings screen.

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)

[noelherrick]

In the method name, the word exchange seems to imply that you are getting rid of the old auth credentials, which you are not, so perhaps lookup_access_token_by_api_secret would be better?