shell: add s3.iam.*, s3.config.show, s3.user.provision; hide legacy commands

Add import/export, configuration summary, and a convenience provisioning
command:

- s3.iam.export: dump full IAM state as JSON (stdout or file)
- s3.iam.import: replace IAM state from a JSON file
- s3.config.show: human-readable summary (users, policies, service
  accounts, groups with status and counts)
- s3.user.provision: one-step user+policy+credentials creation for
  common readonly/readwrite/admin roles

Hide legacy commands from help listing:
- s3.configure: still works but hidden from help output
- s3.bucket.access: still works but hidden from help output

Both hidden commands remain fully functional for existing scripts.

Also adds a Hidden command tag and filters it from printGenericHelp.

weed/shell/command.go

@@ -17,4 +17,5 @@ type CommandTag string
 
 const (
 	ResourceHeavy CommandTag = "resourceHeavy"
+	Hidden        CommandTag = "hidden"
 )