csi: set gRPC authority header for unix domain socket

The go-grpc library used by most CSI plugins doesn't require the
authority header to be set, which violates the HTTP2 spec but doesn't
impact Nomad because both sides of the connection are using the same
library. But plugins written in other languages (`democratic-csi` for
example) may have more strictly conforming gRPC server libraries and
we need to set the authority header manually.

plugins/csi/client.go

@@ -162,6 +162,7 @@ func newGrpcConn(addr string, logger hclog.Logger) (*grpc.ClientConn, error) {
 		grpc.WithInsecure(),
 		grpc.WithUnaryInterceptor(logging.UnaryClientInterceptor(logger)),
 		grpc.WithStreamInterceptor(logging.StreamClientInterceptor(logger)),
+		grpc.WithAuthority("localhost"),
 		grpc.WithDialer(func(target string, timeout time.Duration) (net.Conn, error) {
 			return net.DialTimeout("unix", target, timeout)
 		}),