Add --rollback option to elastic-agent upgrade subcommand #6887
Description
elastic-agent upgrade command will need to have a --rollback option to initiate rollbacks of standalone agents.
This new option needs to be translated in a new parameter in the upgrade request sent to the GRPC server.
(hint: a boolean similar to the one added to the upgrade action described in elastic/fleet-server#4838 would simplify things and keep a consistent interface)