[n7studios]

Summary

Adds the ConvertKit_App_Server class, so Kit WordPress Plugins can connect through the Kit WP App Server and work as Kit App Store apps. Kit for WooCommerce is the first Plugin to use it.

The class provides:

• get_redirect_uri(), get_connect_url() and get_complete_url(): the App Server URLs for the Kit OAuth connect and install flow.
• allowed_redirect_hosts(): lets wp_safe_redirect() redirect to the App Server.
• register_site() and unregister_site(): register the site with the App Server on connect, including the one-time registration token the App Server sends with the authorization code, and unregister it on disconnect.
• heartbeat(): tells the App Server the site is still active.
• send_automation_event(): sends automation events (e.g. abandoned checkout, product purchased) to the App Server, which passes them on to Kit.

Each request includes the Plugin, WordPress and dependency versions passed to the constructor. Requests authenticate with the site secret returned by register_site(). A failed request, or a response with an unexpected status code, returns a WP_Error that includes the HTTP status.

Testing

AppServerTest covers the URLs, allowed redirect hosts, registering (with and without a registration token), unregistering, heartbeat, automation events (with and without context), the versions sent with each request, and unexpected response codes.

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)