[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/generic.py

@@ -153,6 +153,7 @@ class GenericIE(InfoExtractor):
     IE_DESC = 'Generic downloader that works on some sites'
     _VALID_URL = r'.*'
     IE_NAME = 'generic'
+    _NETRC_MACHINE = False  # Supress username warning
     _TESTS = [
         # Direct link to a video
         {