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".
