[management] Let embedding binaries extend the command tree (#7483)

The management binary is embedded by downstream builds that override
server construction via SetNewServer, but the cobra command tree itself
was closed: rootCmd is unexported and fully assembled in init, with no
way to attach additional subcommands. Customize hands the built root
command to a caller-supplied function before Execute, so an embedding
binary can add its own commands next to — or under — the built-in ones,
such as extra administrative helpers beneath the existing admin group.

management/cmd/root.go

@@ -54,6 +54,15 @@ func Execute() error {
 	return rootCmd.Execute()
 }
 
+// Customize hands the fully built root command to fn so an embedding binary
+// can extend or adjust the command tree — most commonly attaching its own
+// subcommands next to (or under) the built-in ones — before calling Execute.
+// The root command is constructed in this package's init, so Customize may be
+// called from the embedding binary's main at any point before Execute.
+func Customize(fn func(root *cobra.Command)) {
+	fn(rootCmd)
+}
+
 func init() {
 	mgmtCmd.Flags().IntVar(&mgmtPort, "port", 80, "server port to listen on (defaults to 443 if TLS is enabled, 80 otherwise")
 	mgmtCmd.Flags().BoolVar(&disableLegacyManagementPort, "disable-legacy-port", false, "disabling the old legacy port (33073)")