The /_ws_tunnel endpoint pipes a WebSocket into the full gRPC service. On a
plaintext loopback gateway any web page could open it, since browsers do not
apply CORS to WebSocket upgrades. Mount the tunnel only when
enable_websocket_tunnel is set (config file, --enable-websocket-tunnel, or
OPENSHELL_ENABLE_WEBSOCKET_TUNNEL; server.enableWebsocketTunnel in Helm).
BREAKING CHANGE: gateways behind an authenticating edge proxy must set
enable_websocket_tunnel = true for CLI edge-tunnel connections.
Signed-off-by: Drew Newberry <anewberry@nvidia.com>