Add JSON endpoint for CAPTCHA rendering

To be used inside WP/WC registration form to construct URL for CAPTCHA page.

[MAILPOET-6325]

mailpoet/lib/API/JSON/Endpoint.php

@@ -2,6 +2,7 @@
 
 namespace MailPoet\API\JSON;
 
+use MailPoet\API\JSON\v1\RedirectResponse;
 use MailPoet\Config\AccessControl;
 
 abstract class Endpoint {
@@ -41,6 +42,10 @@ abstract class Endpoint {
     return new ErrorResponse($errors, $meta, Response::STATUS_BAD_REQUEST);
   }
 
+  public function redirectResponse($url) {
+    return new RedirectResponse($url);
+  }
+
   public function isMethodAllowed($name, $type) {
     // Block GET requests on POST endpoints, but allow POST requests on GET endpoints (some plugins
     // change REQUEST_METHOD to POST on GET requests, which caused them to be blocked)