This allows the user to do 2 things.

1. Change the /purge suffix that nginx uses by default:

add_filter('rt_nginx_helper_purge_url_path', function () {
    return 'custompurgepath';
});

2. Allow changing the full purge url / domain + suffix at once:

add_filter('rt_nginx_helper_purge_url_base', function () {
    return 'https://custompurgedomain.com/purge';
});

Why both filters? When working with nginx-helper I noticed I often have to change nginx paths algorithm to comply with custom scripting or server changes on our side.
The second filter helps when even the full purge url is different, because of internal networking. Although this happens seldom, the filter helps to mitigate these issues.

Would be great to be able to override it this way instead of hard-coding the changes in 👍