shared/att: Implement write handler and bt_att_send.
shared/att: Implement write handler and bt_att_send.

This patch implements the write handler logic, including the way send
operations are process from the various internal queues. Added PDU
encoding for the Exchange MTU request.

pstglia/external-bluetooth-bluez,mapfau/bluez,ComputeCycles/bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,mapfau/bluez,mapfau/bluez,silent-snowman/bluez,ComputeCycles/bluez,silent-snowman/bluez,mapfau/bluez,silent-snowman/bluez,silent-snowman/bluez,ComputeCycles/bluez