[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/eroprofile.py

@@ -39,11 +39,7 @@ class EroProfileIE(InfoExtractor):
         'skip': 'Requires login',
     }]
 
-    def _login(self):
-        (username, password) = self._get_login_info()
-        if username is None:
-            return
-
+    def _perform_login(self, username, password):
         query = compat_urllib_parse_urlencode({
             'username': username,
             'password': password,
@@ -62,9 +58,6 @@ class EroProfileIE(InfoExtractor):
             r'<script[^>]+?src="([^"]+)"', login_page, 'login redirect url')
         self._download_webpage(redirect_url, None, False)
 
-    def _real_initialize(self):
-        self._login()
-
     def _real_extract(self, url):
         display_id = self._match_id(url)