TLS support for GRPC in server and client

[ehazlett]

This adds TLS support for the GRPC backend. The following options are supported:

• TLSServerCertificate: (required) path to the certificate for the server
• TLSServerKey: (required) path to the key for the server
• TLSClientCertificate: (required) path to the client certificate
• TLSClientKey: (optional) path to the key for the client
• TLSInsecureSkipVerify: boolean to disable tls verification

Note: for a quick setup you can generate a self-signed certificate and use the cert and key for both the server and client as long as verification is disabled.

Example Config

    "TLSServerCertificate": "/etc/certs/server.crt",
    "TLSServerKey": "/etc/certs/server.key",
    "TLSClientCertificate": "/etc/certs/client-00.crt",
    "TLSClientKey": "/etc/certs/client-00.key",
    "TLSInsecureSkipVerify": true,

If the options are left as empty strings ("") or removed from the config, TLS is disabled.