parquet_pushdown(M0-C5): add weed pushdown.ping smoke command

Add weed/command/pushdown_ping.go that dials a pushdown daemon over
gRPC, calls Ping with a 5s timeout, and prints the daemon's reported
version and trust mode. Used both as a deployment smoke test and by
the M0 integration test.

Default -server=localhost:18888 matches the daemon's default listen
port. -tls toggles between insecure and the security/grpc.client
TLS config; default insecure mirrors the iam command's pattern for
local-development smoke tests.

Naming follows the existing dotted convention (mq.agent, mq.broker,
mq.kafka.gateway): the daemon is "weed pushdown", the smoke test is
"weed pushdown.ping".

weed/command/command.go

@@ -38,6 +38,7 @@ var Commands = []*Command{
 	cmdMqBroker,
 	cmdMqKafkaGateway,
 	cmdPushdown,
+	cmdPushdownPing,
 	cmdS3,
 	cmdScaffold,
 	cmdServer,