util: add syspolicy package (#9550)

Add a more generalized package for getting policies.
Updates tailcale/corp#10967

Signed-off-by: Claire Wang <claire@tailscale.com>
Co-authored-by: Adrian Dewhurst <adrian@tailscale.com>

util/syspolicy/handler_windows.go

@@ -0,0 +1,32 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package syspolicy
+
+import (
+	"errors"
+
+	"tailscale.com/util/winutil"
+)
+
+type windowsHandler struct{}
+
+func init() {
+	RegisterHandler(windowsHandler{})
+}
+
+func (windowsHandler) ReadString(key string) (string, error) {
+	s, err := winutil.GetPolicyString(key)
+	if errors.Is(err, winutil.ErrNoValue) {
+		err = ErrNoSuchKey
+	}
+	return s, err
+}
+
+func (windowsHandler) ReadUInt64(key string) (uint64, error) {
+	value, err := winutil.GetPolicyInteger(key)
+	if errors.Is(err, winutil.ErrNoValue) {
+		err = ErrNoSuchKey
+	}
+	return value, err
+}