There is already a header for that: List-Unsubscribe with the URL, and the List-Unsubscribe-Post to support one-click unsubscribes, which Google and Yahoo began

There is already a header for that: List-Unsubscribe with the URL, and the List-Unsubscribe-Post to support one-click unsubscribes, which Google and Yahoo began enforcing for bulk senders in February this year.