[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_test.go

@@ -0,0 +1,42 @@
+package cmd
+
+import (
+	"testing"
+
+	"github.com/spf13/cobra"
+)
+
+// TestCustomize verifies an embedding binary can extend the command tree: a
+// top-level command attached through the hook, and a subcommand attached under
+// the built-in admin group, are both resolvable exactly as Execute would
+// resolve them.
+func TestCustomize(t *testing.T) {
+	topLevel := &cobra.Command{Use: "some-extra", RunE: func(*cobra.Command, []string) error { return nil }}
+	nested := &cobra.Command{Use: "cluster", RunE: func(*cobra.Command, []string) error { return nil }}
+
+	Customize(func(root *cobra.Command) {
+		root.AddCommand(topLevel)
+		for _, c := range root.Commands() {
+			if c.Name() == "admin" {
+				c.AddCommand(nested)
+				return
+			}
+		}
+		t.Fatal("admin command not found in the root tree")
+	})
+	t.Cleanup(func() {
+		rootCmd.RemoveCommand(topLevel)
+		for _, c := range rootCmd.Commands() {
+			if c.Name() == "admin" {
+				c.RemoveCommand(nested)
+			}
+		}
+	})
+
+	if found, _, err := rootCmd.Find([]string{"some-extra"}); err != nil || found != topLevel {
+		t.Fatalf("top-level command not resolvable: found=%v err=%v", found, err)
+	}
+	if found, _, err := rootCmd.Find([]string{"admin", "cluster"}); err != nil || found != nested {
+		t.Fatalf("nested admin subcommand not resolvable: found=%v err=%v", found, err)
+	}
+}