sd-network: Introduce API to access DHCP4 client ID

src/libsystemd/sd-network/sd-network.c

@@ -168,6 +168,10 @@ _public_ int sd_network_link_get_address_state(int ifindex, char **state) {
         return network_link_get_string(ifindex, "ADDRESS_STATE", state);
 }
 
+_public_ int sd_network_link_get_dhcp4_client_id_string(int ifindex, char **client_id) {
+        return network_link_get_string(ifindex, "DHCP4_CLIENT_ID", client_id);
+}
+
 _public_ int sd_network_link_get_required_for_online(int ifindex) {
         _cleanup_free_ char *s = NULL;
         int r;