net: virtual: Add capability flag for VPN type interface

Allow user to mark the virtual interface as a VPN one.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>

include/zephyr/net/virtual.h

@@ -46,6 +46,9 @@ enum virtual_interface_caps {
 	/** Virtual Ethernet bridge interface. */
 	VIRTUAL_INTERFACE_BRIDGE = BIT(3),
 
+	/** VPN interface */
+	VIRTUAL_INTERFACE_VPN = BIT(4),
+
 /** @cond INTERNAL_HIDDEN */
 	/* Marker for capabilities - must be at the end of the enum.
 	 * It is here because the capability list cannot be empty.