core/gatt: Support ReadValue for characteristics
core/gatt: Support ReadValue for characteristics

This patch adds support for obtaining the characteristic value from an
external application during a read procedure.

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