Add a juju subcommand for backup.

I've added a new juju subcommand "backups".  The command is itself a "supercommand", meaning it has its own subcommands.  This patch adds a single subcommand "create" (i.e. "juju backups create") which causes a new state backup to get created and stored, and prints the new backup ID.

This command is not yet a replacement for the "juju backup" command (which is implemented as a plugin).  That cannot happen until we add a "juju backups download" command, or at least a --download option to "juju backups create".