feature/clientupdate: move clientupdate to a modular feature, disabled for tsnet

Updates #12614

Change-Id: I5f685dec84a5396b7c2b66f2788ae3d286e1ddc6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/hooks.go

@@ -0,0 +1,17 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package feature
+
+// HookCanAutoUpdate is a hook for the clientupdate package
+// to conditionally initialize.
+var HookCanAutoUpdate Hook[func() bool]
+
+// CanAutoUpdate reports whether the current binary is built with auto-update
+// support and, if so, whether the current platform supports it.
+func CanAutoUpdate() bool {
+	if f, ok := HookCanAutoUpdate.GetOk(); ok {
+		return f()
+	}
+	return false
+}