[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/bbc.py

@@ -264,11 +264,7 @@ class BBCCoUkIE(InfoExtractor):
             'only_matching': True,
         }]
 
-    def _login(self):
-        username, password = self._get_login_info()
-        if username is None:
-            return
-
+    def _perform_login(self, username, password):
         login_page = self._download_webpage(
             self._LOGIN_URL, None, 'Downloading signin page')
 
@@ -294,9 +290,6 @@ class BBCCoUkIE(InfoExtractor):
                     'Unable to login: %s' % error, expected=True)
             raise ExtractorError('Unable to log in')
 
-    def _real_initialize(self):
-        self._login()
-
     class MediaSelectionError(Exception):
         def __init__(self, id):
             self.id = id