Bluetooth: GATT: Add initializer for GATT CCC

Add GATT initializer macro for GATT CCC and allow the
BT_GATT_CCC_MANAGED to accept an already initialized CCC user data.
This allows the application to specify the storage location of the CCC
user data.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>