Add generate private-key command

This commit adds a command to generate a private key for headscale.

Mostly useful for systems were you drive the deployment from another
machine and use a secret management system.

cmd/headscale/cli/generate.go

@@ -0,0 +1,41 @@
+package cli
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+	"tailscale.com/types/key"
+)
+
+func init() {
+	rootCmd.AddCommand(generateCmd)
+	generateCmd.AddCommand(generatePrivateKeyCmd)
+}
+
+var generateCmd = &cobra.Command{
+	Use:   "generate",
+	Short: "Generate commands",
+}
+
+var generatePrivateKeyCmd = &cobra.Command{
+	Use:   "private-key",
+	Short: "Generate a private key for the headscale server",
+	Run: func(cmd *cobra.Command, args []string) {
+		output, _ := cmd.Flags().GetString("output")
+		machineKey := key.NewMachine()
+
+		machineKeyStr, err := machineKey.MarshalText()
+		if err != nil {
+			ErrorOutput(
+				err,
+				fmt.Sprintf("Error getting machine key from flag: %s", err),
+				output,
+			)
+		}
+
+		SuccessOutput(map[string]string{
+			"private_key": string(machineKeyStr),
+		},
+			string(machineKeyStr), output)
+	},
+}