[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/canvas.py

@@ -274,14 +274,7 @@ class VrtNUIE(GigyaBaseIE):
     _APIKEY = '3_0Z2HujMtiWq_pkAjgnS2Md2E11a1AwZjYiBETtwNE-EoEHDINgtnvcAOpNgmrVGy'
     _CONTEXT_ID = 'R3595707040'
 
-    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):
         auth_info = self._gigya_login({
             'APIKey': self._APIKEY,
             'targetEnv': 'jssdk',