[feature] Option to set x509 extensions on the Automatic Tunnel Provisioning certificates

[kosli]

As stated in #743 it would be good to have the possibility to set x509 extensions on the automatic tunnel provisioning certificates.

e.g. I use the certificate not just as client certificates to connect to the openwisp server but also as server certificates for clients connecting to the openwrt router devices. -> and to "harden" the openvpn connection i use remote-cert-tls server option (see the OpenVPN manual)

Thanks