tools: Add Get/Set descriptor
tools: Add Get/Set descriptor

This patch adds get and set callbacks to allow managing "Value" property
of the user defined descriptor related to IAS Alert Level.

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