rest: add a Signer callback
rest: add a Signer callback

seuffert/rclone,olihey/rclone,yonjah/rclone,olihey/rclone,olihey/rclone,seuffert/rclone,ncw/rclone,ncw/rclone,yonjah/rclone,seuffert/rclone,ncw/rclone,B4dM4n/rclone,B4dM4n/rclone,B4dM4n/rclone,yonjah/rclone,seuffert/rclone,B4dM4n/rclone,ncw/rclone,yonjah/rclone,olihey/rclone