Add endpoint what will return the list of app ENVs

Fixes #1964

pages/api/config.ts

+import type { NextApiRequest, NextApiResponse } from 'next';
+
+export default async function configHandler(req: NextApiRequest, res: NextApiResponse) {
+  // eslint-disable-next-line no-restricted-properties
+  const publicEnvs = Object.entries(process.env)
+    .filter(([ key ]) => key.startsWith('NEXT_PUBLIC_'))
+    .sort(([ keyA ], [ keyB ]) => keyA.localeCompare(keyB))
+    .reduce((result, [ key, value ]) => {
+      result[key] = value || '';
+      return result;
+    }, {} as Record<string, string>);
+
+  res.status(200).json({
+    envs: publicEnvs,
+  });
+}