net: lwm2m: move to flat buffers

As part of the migration from net_app APIs to socket APIs, let's
stop referencing the net_pkt fragments throughout the LwM2M library.

Establish a msg_data flat buffer inside lwm2m_message and use that
instead.

NOTE: As a part of this change we remove the COAP_NET_PKT setting.
The COAP library reverts to COAP_SOCK behavior.

This doesn't mean we use sockets in LwM2M (yet), it only means we
use the socket-compatible COAP library which parses flat buffers
instead of net_pkt fragments.

Signed-off-by: Michael Scott <mike@foundries.io>