Improve get_resource()

[n7studios]

Summary

Improves the get_resource() API function, with PHP 8.2 compatibility for fetching legacy forms and legacy landing pages, handling encoding (as used in the WordPress Libraries code)

Removes outdated (and now unused) dependencies and not recommended file_get_contents() calls.

Testing

• testGetResourceLegacyForm: Tests that fetching a legacy form's HTML works with correct encoding
• testGetResourceLandingPage: Tests that fetching a landing page's HTML works with correct encoding
• testGetResourceLegacyLandingPage: Tests that fetching a legacy landing page's HTML works with correct encoding

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)