[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/linkedin.py

@@ -25,12 +25,9 @@ class LinkedInBaseIE(InfoExtractor):
     _NETRC_MACHINE = 'linkedin'
     _logged_in = False
 
-    def _real_initialize(self):
+    def _perform_login(self, username, password):
         if self._logged_in:
             return
-        email, password = self._get_login_info()
-        if email is None:
-            return
 
         login_page = self._download_webpage(
             self._LOGIN_URL, None, 'Downloading login page')
@@ -39,7 +36,7 @@ class LinkedInBaseIE(InfoExtractor):
             default='https://www.linkedin.com/uas/login-submit', group='url'))
         data = self._hidden_inputs(login_page)
         data.update({
-            'session_key': email,
+            'session_key': username,
             'session_password': password,
         })
         login_submit_page = self._download_webpage(