Adds support for receiving Multicast Group addresses + IGMP ( IPv4 only )

Adds 2 function pointers to the network interface struct that handle adding and removing  multicast MAC addresses.
Updates IGMP to use function pointers through the network interface.
Makes the Add/Remove Multicast functions private to NetworkInterface.c They are now used through pointers in the NetworkInterface_t struct.
Improves the SAME70 driver to handle adding/removing muticast MAC addresses

Author: Emil Popov

source/FreeRTOS_DNS_Networking.c

@@ -84,6 +84,11 @@
              * going to be '0' i.e. success. Thus, return value is discarded */
             ( void ) FreeRTOS_setsockopt( xSocket, 0, FREERTOS_SO_SNDTIMEO, &( uxWriteTimeOut_ticks ), sizeof( TickType_t ) );
             ( void ) FreeRTOS_setsockopt( xSocket, 0, FREERTOS_SO_RCVTIMEO, &( uxReadTimeOut_ticks ), sizeof( TickType_t ) );
+            #if ( ipconfigSUPPORT_IP_MULTICAST != 0 )
+                /* Since this socket may be used for LLMNR or mDNS, set the multicast TTL to 1. */
+                uint8_t ucMulticastTTL = 1;
+                ( void ) FreeRTOS_setsockopt( xSocket, 0, FREERTOS_SO_IP_MULTICAST_TTL, &( ucMulticastTTL ), sizeof( uint8_t ) );
+            #endif
         }
 
         return xSocket;