[fansly] add initial support (#4401)

- only posts and creator posts work
- 'home' and 'list' extractors do nothing atm
- needs 'authorization' header as 'token' config value
  to access locked content
- needs yt-dlp to download 1080p videos

docs/gallery-dl.conf

@@ -328,6 +328,10 @@
             "fee-max" : null,
             "metadata": false
         },
+        "fansly":
+        {
+            "token": ""
+        },
         "flickr":
         {
             "access-token"       : null,