parquet_pushdown(M0-C4): wire the weed pushdown subcommand

Add weed/command/pushdown.go that parses CLI flags (-filer, -ip,
-port, -trust, -dev.connector_trusted_ack), assembles a daemon.Config,
and calls daemon.Run. Default port 18888 is unused elsewhere in the
SeaweedFS port table and is far enough above the filer/master/volume
ports to be unsurprising in operations. Long help spells out the two
trust modes and points the operator at PARQUET_PUSHDOWN_DESIGN.md and
PARQUET_PUSHDOWN_DEV_PLAN.md for context.

Register cmdPushdown in weed/command/command.go's Commands list so
"weed pushdown" appears in the top-level help. The command is gated
"<WIP>" until later milestones land actual pruning behavior.

weed/command/command.go

@@ -37,6 +37,7 @@ var Commands = []*Command{
 	cmdMqAgent,
 	cmdMqBroker,
 	cmdMqKafkaGateway,
+	cmdPushdown,
 	cmdS3,
 	cmdScaffold,
 	cmdServer,