Add Creator Network Recommendations

Summary

Adds an option to enable the Creator Network Recommendations to a WPForms Form: Screenshot 2023-07-19 at 15 39 53

When enabled, the modal is displayed when the form is submitted: Screenshot 2023-07-19 at 15 39 37

Several requirements must be met for this to work:

If the API endpoint enabled property returns false, the user is prompted to upgrade their ConvertKit Plan.

Screenshot 2023-07-19 at 15 41 46

When configuring the WPForms Form, its AJAX setting must be enabled (by default, it is enabled):

Screenshot 2023-07-19 at 15 42 18

If disabled, WPForms will submit the form as a new request, reloading the page, meaning that the modal will not display.

Tests are supplied in this PR for ease of review, given the amount of code here.

[x] I have written a test and included it in this PR
[x] I have run all tests and they pass
[x] The code passes when running the PHP CodeSniffer
[x] Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
[x] Security and Sanitization requirements have been followed
[x] 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)