EMQX v5 add a new zone and link a SSL listener to it

emqx-conf-cert-md5-as-user-name.conf

## Verified in EMQX 5.3.0
## listeners in addition to the default ones
listeners {
    ssl.certhash {
        enable = true
        # bind to port 8884
        bind = 8884
        # link it to zone named 'certhash'
        zone = certhash
        ssl_options {
            # force the clients to send certificate
            verify = verify_peer
            fail_if_no_peer_cert = true
        }
    }
}

## zones in addition to 'default'
zones {
    # the new zone is named 'certhash'
    certhash {
        mqtt {
            # md5 hash of the DER format (but not PEM) of the client X.509 certificate
            peer_cert_as_username = md5
        }
    }
}