Add beforeLogin trigger with support for auth providers (#5445)

* Add beforeLogin trigger with support for auth providers

* adjust comment that boxed off beforeLogin to a negative use-case only

* add internal error to help future maintainers regarding use of beforeLogin

* let beforeLogin accept className or constructor like other hook types

* add assertions for beforeLogin trigger className validation

src/triggers.js

@@ -3,6 +3,7 @@ import Parse from 'parse/node';
 import { logger } from './logger';
 
 export const Types = {
+  beforeLogin: 'beforeLogin',
   beforeSave: 'beforeSave',
   afterSave: 'afterSave',
   beforeDelete: 'beforeDelete',
@@ -41,6 +42,11 @@ function validateClassNameForTriggers(className, type) {
     // TODO: Allow proper documented way of using nested increment ops
     throw 'Only afterSave is allowed on _PushStatus';
   }
+  if (type === Types.beforeLogin && className !== '_User') {
+    // TODO: check if upstream code will handle `Error` instance rather
+    // than this anti-pattern of throwing strings
+    throw 'Only the _User class is allowed for the beforeLogin trigger';
+  }
   return className;
 }