[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/niconico.py

@@ -183,16 +183,7 @@ class NiconicoIE(InfoExtractor):
         'Origin': 'https://www.nicovideo.jp',
     }
 
-    def _real_initialize(self):
-        self._login()
-
-    def _login(self):
-        username, password = self._get_login_info()
-        # No authentication to be performed
-        if not username:
-            return True
-
-        # Log in
+    def _perform_login(self, username, password):
         login_ok = True
         login_form_strs = {
             'mail_tel': username,