Implements `lttng_event_notifier_notification_{create,destroy}()`

Implements `lttng_event_notifier_notification_{create,destroy}()`

Capture payload will be later added to the lttng_event_notifier_notification object.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I404984c522cdf46c0df7c6ffbbe340d2f56cda2d
Depends-on: lttng-ust: I8423c510bf6af2f9bf85256e8d6f931d36f7054b