Add support for backends which require client certificate (eg. NiFi)
authentication. The `proxy-ssl-secret` k8s annotation references a
secret which is used to authenticate to the backend server. All other
directives fine tune the backend communication.
The following annotations are supported:
* proxy-ssl-secret
* proxy-ssl-ciphers
* proxy-ssl-protocol
* proxy-ssl-verify
* proxy-ssl-verify-depth
