[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/gaia.py

@@ -56,24 +56,22 @@ class GaiaIE(InfoExtractor):
     def _real_initialize(self):
         auth = self._get_cookies('https://www.gaia.com/').get('auth')
         if auth:
-            auth = self._parse_json(
-                compat_urllib_parse_unquote(auth.value),
-                None, fatal=False)
-        if not auth:
-            username, password = self._get_login_info()
-            if username is None:
-                return
-            auth = self._download_json(
-                'https://auth.gaia.com/v1/login',
-                None, data=urlencode_postdata({
-                    'username': username,
-                    'password': password
-                }))
-            if auth.get('success') is False:
-                raise ExtractorError(', '.join(auth['messages']), expected=True)
-        if auth:
+            auth = self._parse_json(compat_urllib_parse_unquote(auth.value), None, fatal=False)
             self._jwt = auth.get('jwt')
 
+    def _perform_login(self, username, password):
+        if self._jwt:
+            return
+        auth = self._download_json(
+            'https://auth.gaia.com/v1/login',
+            None, data=urlencode_postdata({
+                'username': username,
+                'password': password
+            }))
+        if auth.get('success') is False:
+            raise ExtractorError(', '.join(auth['messages']), expected=True)
+        self._jwt = auth.get('jwt')
+
     def _real_extract(self, url):
         display_id, vtype = self._match_valid_url(url).groups()
         node_id = self._download_json(