Add next_cls_cb attribute to PacketListField

  - this feature adds the ability to have a PacketListField
    of heterogeneous Packet types with dynamic discovery
    of the next type. This discovery can be based on any elements
    including previously parsed packets, underlayers, remaining
    bytes (look ahead), and last parsed packet.
  - this feature also adds the ability to parse PacketListFields
    where neither the length nor the number of elements can be
    predicted before parsing. This could be done previously using
    a length_from callback that did significant peeks into the
    string to parse, but it felt clumsy.