Bluetooth: host: directed advertising support

Added implementation to the directed advertising API in the Connection
Management module. Introduced a new connection state for this type of
advertising. The new state is symmetric to the connection state used for
scanning.

Added a new advertising option that can be used to trigger low and high
duty directed advertising. Added macros for default values of
Advertising Parameters, which are used to trigger directed advertising.

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