[update] implement '--update-to'

to switch between update channels (stable or dev (alias nightly & master))
or to install a specific version

Examples:
--update-to dev
--update-to dev@2024.05.25
--update-to v1.25.2

gallery_dl/option.py

@@ -179,6 +179,11 @@ def build_parser():
             dest="update", action="store_const", const="latest",
             help="Update to the latest version",
         )
+        general.add_argument(
+            "--update-to",
+            dest="update", metavar="[CHANNEL@]TAG",
+            help="Upgrade/downgrade to a specific version",
+        )
         general.add_argument(
             "--update-check",
             dest="update", action="store_const", const="check",