shared/gatt-server: Implement "Prepare Write" request.
shared/gatt-server: Implement "Prepare Write" request.

This patch add support for handling ATT "Prepare Write" requests for the
server role.

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