feat: 🎸 add API and mock support for primary auth methods

addons/api/addon/models/auth-method.js

@@ -1,3 +1,6 @@
 import GeneratedAuthMethodModel from '../generated/models/auth-method';
+import { equal } from '@ember/object/computed';
 
-export default class AuthMethodModel extends GeneratedAuthMethodModel {}
+export default class AuthMethodModel extends GeneratedAuthMethodModel {
+  @equal('primary', true) isPrimary;
+}