[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/tubitv.py

@@ -54,10 +54,7 @@ class TubiTvIE(InfoExtractor):
         },
     }]
 
-    def _login(self):
-        username, password = self._get_login_info()
-        if username is None:
-            return
+    def _perform_login(self, username, password):
         self.report_login()
         form_data = {
             'username': username,
@@ -72,9 +69,6 @@ class TubiTvIE(InfoExtractor):
             raise ExtractorError(
                 'Login failed (invalid username/password)', expected=True)
 
-    def _real_initialize(self):
-        self._login()
-
     def _real_extract(self, url):
         video_id = self._match_id(url)
         video_data = self._download_json(