whereami: add support for secure gRPC port using ENV variables

[gbrayut]

Whereami currently supports gRPC with grpc.health.v1.Health checks but only using insecure non-tls port.

Google Cloud Load Balancers require end-to-end encryption so it can use ALPN to upgrade backend connections to HTTP/2. It would be useful if whereami could support secure port gRPC instead of insecure using environment variables that indicate which certificate and key should be used by the gRPC server. We can then use cert-manager or SPIFFE+CSI to inject the certificates into the pod.

I discussed this briefly with @theemadnes and he requested this issue be assigned to him.