Creating a user for pg_start_backup

Dear all,

I'm setting up hot backups on my database server. As such, I'd like to set up a
Postgres user that has access to only pg_start_backup and pg_stop_backup.

I'm unable to work out how to do this with the various GRANT options. Can someone
point me in the right direction please? Or is there a better way to achieve this,
rather than having a dedicated user?

Thanks,

Andy