Don't pass vars to frontend inside HTML

[erommel]

Currently, vars for payment buttons are generated on server side and passed to frontend JS via wp_localize_script function (or analogue), which puts those in the HTML code of the page. This can cause some random issues on sites with excessive caching.

To prevent that, an experimental option can be added. When enabled, plugin won't pass the vars inside HTML page, but will be requesting those via AJAX on page load.

This has some disadvantages though. On slow websites, it would increase the time required for payment buttons (and payment pop-ups) to be operational.

[Arsenal21]

An experimental option sounds good for this.