net: tcp2: Implement TCP data sending

Since TCP needs to be able to decide how much data is to be sent
in a TCP segment and when the segment needs to be sent, the TCP
data buffer needs to be passed down to the TCP stack. For tcp2
this causes a new function handling the data as a buffer or as an
iov to be implemented and only that function is called when
sending data out via the new TCP stack. net_tcp_send_data() is
invoked as the caller expects to be informed when the data has
been sent.

For the current stack keep the sending functions as is.

Signed-off-by: Patrik Flykt <patrik.flykt@intel.com>
Signed-off-by: Oleg Zhurakivskyy <oleg.zhurakivskyy@intel.com>