util/syspolicy: add ReadStringArray interface (#11857)

Fixes tailscale/corp#19459

This PR adds the ability for users of the syspolicy handler to read string arrays from the MDM solution configured on the system.

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

util/syspolicy/handler_windows.go

@@ -93,3 +93,13 @@ func (windowsHandler) ReadBoolean(key string) (bool, error) {
 	}
 	return value != 0, err
 }
+
+func (windowsHandler) ReadStringArray(key string) ([]string, error) {
+	value, err := winutil.GetPolicyStringArray(key)
+	if errors.Is(err, winutil.ErrNoValue) {
+		err = ErrNoSuchKey
+	} else if err != nil {
+		windowsErrors.Add(1)
+	}
+	return value, err
+}