[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/linuxacademy.py

@@ -75,14 +75,7 @@ class LinuxAcademyIE(InfoExtractor):
     _CLIENT_ID = 'KaWxNn1C2Gc7n83W9OFeXltd8Utb5vvx'
     _NETRC_MACHINE = 'linuxacademy'
 
-    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):
         def random_string():
             return ''.join([
                 random.choice('0123456789ABCDEFGHIJKLMNOPQRSTUVXYZabcdefghijklmnopqrstuvwxyz-._~')