Bluetooth: Gatt: Automatic discover of CCC when subscribing

Added a feature that allows an application to request
automatic discovery of the CCCD handle when subscriping
to a characteristic. In order to preserver RAM, the application
is expected to provide the discovery parameters, and it's up
to the application whether or not it wants to reuse the
discovery parameters or use one for each characteristic.

Signed-off-by: Emil Gydesen <emil_gydesen@bose.com>