shared/gatt-client: Implement descriptor discovery.
shared/gatt-client: Implement descriptor discovery.

This patch implements characteristic descriptor discovery as part of the client
initialization flow.

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