Bluetooth: Host: Add option to use GATT over EATT

A channel option field has been added to the bt_gatt_*_params structs.
This allows the application to choose wether Unenhanced ATT, Enhanced
ATT, or Any ATT channel shall be used for the
request/command/indication/notification.

Signed-off-by: Herman Berget <herman.berget@nordicsemi.no>