Override server name when validating TLS cert

Sometimes a trusted cert's hostname cannot be identical to the DNS
label. This flag enables to override the hostname on validation, such
that one is not forced to use --insecure.

cmd/grpcurl/grpcurl.go

@@ -76,6 +76,7 @@ var (
 		`When describing messages, show a JSON template for the message type.`)
 	verbose = flag.Bool("v", false,
 		`Enable verbose output.`)
+	serverName = flag.String("servername", "", "Override servername when validating TLS certificate.")
 )
 
 func init() {
@@ -233,6 +234,9 @@ func main() {
 			if err != nil {
 				fail(err, "Failed to configure transport credentials")
 			}
+			if *serverName != "" {
+				creds.OverrideServerName(*serverName)
+			}
 		}
 		cc, err := grpcurl.BlockingDial(ctx, target, creds, opts...)
 		if err != nil {