[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/lynda.py

@@ -21,9 +21,6 @@ class LyndaBaseIE(InfoExtractor):
     _ACCOUNT_CREDENTIALS_HINT = 'Use --username and --password options to provide lynda.com account credentials.'
     _NETRC_MACHINE = 'lynda'
 
-    def _real_initialize(self):
-        self._login()
-
     @staticmethod
     def _check_error(json_string, key_or_keys):
         keys = [key_or_keys] if isinstance(key_or_keys, compat_str) else key_or_keys
@@ -32,7 +29,7 @@ class LyndaBaseIE(InfoExtractor):
             if error:
                 raise ExtractorError('Unable to login: %s' % error, expected=True)
 
-    def _login_step(self, form_html, fallback_action_url, extra_form_data, note, referrer_url):
+    def _perform_login_step(self, form_html, fallback_action_url, extra_form_data, note, referrer_url):
         action_url = self._search_regex(
             r'<form[^>]+action=(["\'])(?P<url>.+?)\1', form_html,
             'post url', default=fallback_action_url, group='url')
@@ -55,11 +52,7 @@ class LyndaBaseIE(InfoExtractor):
 
         return response, action_url
 
-    def _login(self):
-        username, password = self._get_login_info()
-        if username is None:
-            return
-
+    def _perform_login(self, username, password):
         # Step 1: download signin page
         signin_page = self._download_webpage(
             self._SIGNIN_URL, None, 'Downloading signin page')