Add API about Notification consumer unsubscription provider.

Add API about Notification consumer unsubscription provider.

Added API :
NSResult NSUnsubscribe(const char * providerId);

When added api is called, consumer service will send cancel subscription
and remove information of parameter provider Id.

Change-Id: I2536283e5f0c91477ed4364a1ee230b0ff97b308
Signed-off-by: KIM JungYong <jyong2.kim@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/16249
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>
Reviewed-by: Uze Choi <uzchoi@samsung.com>