[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/adn.py

@@ -126,10 +126,7 @@ def _get_subtitles(self, sub_url, video_id):
             }])
         return subtitles
 
-    def _real_initialize(self):
-        username, password = self._get_login_info()
-        if not username:
-            return
+    def _perform_login(self, username, password):
         try:
             access_token = (self._download_json(
                 self._API_BASE_URL + 'authentication/login', None,