Add support for listening on HTTP and HTTPS

[tsandall]

Currently OPA can listen for HTTP or HTTPS connections but not both. We should update OPA so that users can specify --insecure-addr and --addr to make OPA listen on both HTTP and HTTPS respectively.

This will be useful if OPA is deployed as a sidecar container. The app can contact OPA over localhost while other callers can potentially access OPA's APIs securely via HTTPS.