net: openthread: add option for setting Thread v1.3

Make Thread v1.3 configurable.

Signed-off-by: Lukasz Maciejonczyk <lukasz.maciejonczyk@nordicsemi.no>
diff --git a/subsys/net/l2/openthread/Kconfig.features b/subsys/net/l2/openthread/Kconfig.features
index c80bd28..57b25c5 100644
--- a/subsys/net/l2/openthread/Kconfig.features
+++ b/subsys/net/l2/openthread/Kconfig.features
@@ -13,12 +13,15 @@
 	bool "Version 1.1"
 config OPENTHREAD_THREAD_VERSION_1_2
 	bool "Version 1.2"
+config OPENTHREAD_THREAD_VERSION_1_3
+	bool "Version 1.3"
 endchoice
 
 config OPENTHREAD_THREAD_VERSION
 	string
 	default "1.1" if OPENTHREAD_THREAD_VERSION_1_1
 	default "1.2" if OPENTHREAD_THREAD_VERSION_1_2
+	default "1.3" if OPENTHREAD_THREAD_VERSION_1_3
 	default "unknown"
 
 config OPENTHREAD_BACKBONE_ROUTER