cmd/tailscale: define CLI tools to manipulate macOS network and system extensions (#14727)

Updates tailscale/corp#25278

Adds definitions for new CLI commands getting added in v1.80. Refactors some pre-existing CLI commands within the `configure` tree to clean up code.

Signed-off-by: Andrea Gottardo <andrea@gottardo.me>

cmd/tailscale/cli/configure_apple-all.go

@@ -0,0 +1,11 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package cli
+
+import "github.com/peterbourgon/ff/v3/ffcli"
+
+var (
+	maybeSysExtCmd    func() *ffcli.Command // non-nil only on macOS, see configure_apple.go
+	maybeVPNConfigCmd func() *ffcli.Command // non-nil only on macOS, see configure_apple.go
+)