[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/nebula.py

@@ -148,14 +148,12 @@ class NebulaBaseIE(InfoExtractor):
             'creator': episode['channel_title'],
         }
 
-    def _login(self):
+    def _perform_login(self, username=None, password=None):
+        # FIXME: username should be passed from here to inner functions
         self._nebula_api_token = self._retrieve_nebula_api_token()
         self._nebula_bearer_token = self._fetch_nebula_bearer_token()
         self._zype_access_token = self._fetch_zype_access_token()
 
-    def _real_initialize(self):
-        self._login()
-
 
 class NebulaIE(NebulaBaseIE):
     _VALID_URL = r'https?://(?:www\.)?(?:watchnebula\.com|nebula\.app)/videos/(?P<id>[-\w]+)'