fix: implement 'login' event for net.ClientRequest (#21096)

docs/api/app.md

@@ -323,8 +323,8 @@ Returns:
   * `port` Integer
   * `realm` String
 * `callback` Function
-  * `username` String
-  * `password` String
+  * `username` String (optional)
+  * `password` String (optional)
 
 Emitted when `webContents` wants to do basic auth.
 
@@ -341,6 +341,10 @@ app.on('login', (event, webContents, details, authInfo, callback) => {
 })
 ```
 
+If `callback` is called without a username or password, the authentication
+request will be cancelled and the authentication error will be returned to the
+page.
+
 ### Event: 'gpu-info-update'
 
 Emitted whenever there is a GPU info update.