Add Usage Tracking Checkbox

[n7studios]

Summary

Adds a Usage Tracking checkbox to the Plugin settings and Setup Wizard screens, that creators may opt in to, so we can later track the Plugins and Themes installed on sites with the Kit Plugin installed, to help improve compatibility and migrations.

We can't automatically collect this without consent:
https://developer.wordpress.org/plugins/wordpress-org/detailed-plugin-guidelines/#7-plugins-may-not-track-users-without-their-consent

We don't have an API endpoint to track this on the wordpress namespace, but we can add one later, and then start collecting some data.

Testing

• testEnableAndDisableUsageTrackingSetting: Test that no PHP errors or notices are displayed on the Plugin's Setting screen when the Usage Tracking settings is unchecked, and that CSS is output on the frontend web site.
• testSetupWizardUsageTrackingSetting: Test that the Setup Wizard > Usage Tracking setting is honored.

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)

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