networkd: introduce netdev "Netdevsim" Driver

This "netdevsim" as implied by the name is a tool for network developers and is a simulator.
This simulated networking device is used for testing various networking APIs and at this time
is particularly focused on testing hardware offloading related interfaces.

src/libsystemd/sd-netlink/netlink-types.c

@@ -325,6 +325,7 @@ static const char* const nl_union_link_info_data_table[] = {
         [NL_UNION_LINK_INFO_DATA_GENEVE] = "geneve",
         [NL_UNION_LINK_INFO_DATA_VXCAN] = "vxcan",
         [NL_UNION_LINK_INFO_DATA_WIREGUARD] = "wireguard",
+        [NL_UNION_LINK_INFO_DATA_NETDEVSIM] = "netdevsim",
 };
 
 DEFINE_STRING_TABLE_LOOKUP(nl_union_link_info_data, NLUnionLinkInfoData);

src/libsystemd/sd-netlink/netlink-types.h

@@ -82,6 +82,7 @@ typedef enum NLUnionLinkInfoData {
         NL_UNION_LINK_INFO_DATA_GENEVE,
         NL_UNION_LINK_INFO_DATA_VXCAN,
         NL_UNION_LINK_INFO_DATA_WIREGUARD,
+        NL_UNION_LINK_INFO_DATA_NETDEVSIM,
         _NL_UNION_LINK_INFO_DATA_MAX,
         _NL_UNION_LINK_INFO_DATA_INVALID = -1
 } NLUnionLinkInfoData;