generate user key for new cluster and add it to peers on bootstrap machine

internal/cli/config/cluster.go

@@ -6,4 +6,6 @@ type Cluster struct {
 	Name     string              `toml:"-"`
 	Machines []MachineConnection `toml:"machines"`
 	Secret   secret.Secret       `toml:"secret"`
+	// UserKey is the user's WireGuard private key used to connect to cluster machines.
+	UserKey secret.Secret `toml:"user_key"`
 }