mgmt: mcumgr: transport: bluetooth: add all gatt permission levels

Added new Kconfigs for defining permission level of GATT
characteristics that are part of the Bluetooth SMP service in the
MCUmgr subsys.

Removed the CONFIG_MCUMGR_TRANSPORT_BT_AUTHEN as the new Kconfig
options are mutually exclusive and need to be groupa as the Kconfig
choice option.

Signed-off-by: Kamil Piszczek <Kamil.Piszczek@nordicsemi.no>