[options] add '--sleep-429' command-line option (#7871)

docs/options.md

@@ -92,6 +92,8 @@
                                 (e.g. 2.7 or 2.0-3.5)
     --sleep-request SECONDS     Number of seconds to wait between HTTP requests
                                 during data extraction
+    --sleep-429 SECONDS         Number of seconds to wait when receiving a '429
+                                Too Many Requests' response
     --sleep-extractor SECONDS   Number of seconds to wait before starting data
                                 extraction for an input URL
     --no-part                   Do not use .part files

gallery_dl/option.py

@@ -538,6 +538,12 @@ def build_parser():
         help=("Number of seconds to wait between HTTP requests "
               "during data extraction"),
     )
+    downloader.add_argument(
+        "--sleep-429",
+        dest="sleep-429", metavar="SECONDS", action=ConfigAction,
+        help=("Number of seconds to wait when receiving a "
+              "'429 Too Many Requests' response"),
+    )
     downloader.add_argument(
         "--sleep-extractor",
         dest="sleep-extractor", metavar="SECONDS", action=ConfigAction,