rp2040/rp2350: support setsockopt(IPPROTO_IP, IP_MULTICAST_TTL, ttl)

[jepler]

This is needed for todbot/CircuitPython_MicroOSC#1

It appears this is enabled at the lwip library level but needs to be enabled at the socketpool layer, a la the lwip_setsockopt_impl implementation which says:

        case IP_MULTICAST_TTL:
          LWIP_SOCKOPT_CHECK_OPTLEN_CONN_PCB_TYPE(sock, optlen, u8_t, NETCONN_UDP);
          udp_set_multicast_ttl(sock->conn->pcb.udp, (u8_t)(*(const u8_t *)optval));
          break;