[feature] Allow changing default certificate settings, e.g. set x509 extensions for automatic tunnel provisioning

To be able to use the OpenVPN remote-cert-tls server option (see the OpenVPN manual), the server certificate needs to be created with an extended key usage of "TLS Web Server Authentication". When manually creating a certificate in OpenWISP, the Extensions field can be used with the below settings to add the needed extended key usage:

[ {
"name": "extendedKeyUsage",
"critical": true,
"value": "serverAuth"
} ]

Unfortunately the extension cannot be set on the certificate created via the "Automatic tunnel provisioning" feature -> it would be great if there would be an option to change the settings of the automatically created certificates. e.g. if one use the automatically created certificates not just for the OpenVPN client but also as a OpenVPN server certificate.

Start by tracing the certificate creation path used by Automatic tunnel provisioning and compare it with the manual certificate Extensions field described in the issue. Done means automatically created certificates accept configurable extension settings and can include extendedKeyUsage=serverAuth for OpenVPN server use.