[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/vk.py

@@ -29,11 +29,7 @@ from .youtube import YoutubeIE
 class VKBaseIE(InfoExtractor):
     _NETRC_MACHINE = 'vk'
 
-    def _login(self):
-        username, password = self._get_login_info()
-        if username is None:
-            return
-
+    def _perform_login(self, username, password):
         login_page, url_handle = self._download_webpage_handle(
             'https://vk.com', None, 'Downloading login page')
 
@@ -57,9 +53,6 @@ class VKBaseIE(InfoExtractor):
             raise ExtractorError(
                 'Unable to login, incorrect username and/or password', expected=True)
 
-    def _real_initialize(self):
-        self._login()
-
     def _download_payload(self, path, video_id, data, fatal=True):
         data['al'] = 1
         code, payload = self._download_json(