Added a feature flag for additional payment methods

ref ENG-812
ref https://linear.app/tryghost/issue/ENG-812

apps/admin-x-settings/src/components/settings/advanced/labs/AlphaFeatures.tsx

@@ -59,6 +59,10 @@ const features = [{
     title: 'NestJS Playground',
     description: 'Wires up the Ghost NestJS App to the Admin API',
     flag: 'NestPlayground'
+},{
+    title: 'Additional payment methods',
+    description: 'Adds support for additional payment methods in Stripe Checkout',
+    flag: 'additionalPaymentMethods'
 }];
 
 const AlphaFeatures: React.FC = () => {

ghost/core/core/shared/labs.js

@@ -36,6 +36,7 @@ const BETA_FEATURES = [
 ];
 
 const ALPHA_FEATURES = [
+    'additionalPaymentMethods',
     'NestPlayground',
     'urlCache',
     'lexicalMultiplayer',