Allow Socket API applications to use IP multicast communication by joining and leaving Multicast groups

Socket API applications shall be able to communicate via IP Multicast in both directions: Sending and Receiving.

The "SOCKET API" interface provides host applications an API to join and leave IP Multicast groups.
The new taglist entry HIL_TAG_LWIP_AMOUNT_SOCKET_API_MULTICAST_GROUPS is added to the firmware that can be used to activate and configure this feature.