[extractor] Add _perform_login function (#2943)

* Adds new functions `_initialize_pre_login` and `_perform_login` as part of the extractor API
* Adds `ie.supports_login` to the public API

yt_dlp/extractor/shahid.py

@@ -79,16 +79,12 @@ class ShahidIE(ShahidBaseIE):
         'only_matching': True
     }]
 
-    def _real_initialize(self):
-        email, password = self._get_login_info()
-        if email is None:
-            return
-
+    def _perform_login(self, username, password):
         try:
             user_data = self._download_json(
                 'https://shahid.mbc.net/wd/service/users/login',
                 None, 'Logging in', data=json.dumps({
-                    'email': email,
+                    'email': username,
                     'password': password,
                     'basic': 'false',
                 }).encode('utf-8'), headers={