[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/afreecatv.py

@@ -184,14 +184,7 @@ class AfreecaTVIE(InfoExtractor):
             video_key['part'] = int(m.group('part'))
         return video_key
 
-    def _real_initialize(self):
-        self._login()
-
-    def _login(self):
-        username, password = self._get_login_info()
-        if username is None:
-            return
-
+    def _perform_login(self, username, password):
         login_form = {
             'szWork': 'login',
             'szType': 'json',