[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/zattoo.py

@@ -25,13 +25,11 @@ class ZattooPlatformBaseIE(InfoExtractor):
     def _host_url(self):
         return 'https://%s' % (self._API_HOST if hasattr(self, '_API_HOST') else self._HOST)
 
-    def _login(self):
-        username, password = self._get_login_info()
-        if not username or not password:
-            self.raise_login_required(
-                'A valid %s account is needed to access this media.'
-                % self._NETRC_MACHINE)
+    def _real_initialize(self):
+        if not self._power_guide_hash:
+            self.raise_login_required('An account is needed to access this media', method='password')
 
+    def _perform_login(self, username, password):
         try:
             data = self._download_json(
                 '%s/zapi/v2/account/login' % self._host_url(), None, 'Logging in',
@@ -52,7 +50,7 @@ class ZattooPlatformBaseIE(InfoExtractor):
 
         self._power_guide_hash = data['session']['power_guide_hash']
 
-    def _real_initialize(self):
+    def _initialize_pre_login(self):
         webpage = self._download_webpage(
             self._host_url(), None, 'Downloading app token')
         app_token = self._html_search_regex(
@@ -72,8 +70,6 @@ class ZattooPlatformBaseIE(InfoExtractor):
                 'format': 'json',
             }))
 
-        self._login()
-
     def _extract_cid(self, video_id, channel_name):
         channel_groups = self._download_json(
             '%s/zapi/v2/cached/channels/%s' % (self._host_url(),