refactor: rewrite create-app-auth-client mutation as REST endpoint

packages/backend/src/controllers/api/v1/admin/apps/create-auth-client.ee.js

@@ -0,0 +1,14 @@
+import { renderObject } from '../../../../../helpers/renderer.js';
+import AppConfig from '../../../../../models/app-config.js';
+
+export default async (request, response) => {
+  const appConfig = await AppConfig.query()
+    .findOne({ key: request.params.appKey })
+    .throwIfNotFound();
+
+  const appAuthClient = await appConfig
+    .$relatedQuery('appAuthClients')
+    .insert(request.body);
+
+  renderObject(response, appAuthClient, { status: 201 });
+};