actions: introduce snapshot session action

This patch introduces the API for the "snapshot session" action.

A snapshot session action is created using the
lttng_action_snapshot_session_create function. It is mandatory to set a
session name using lttng_action_snapshot_session_set_session_name before
using the action in a trigger.

It is possible, but optional, to provide a snapshot name with
lttng_action_snapshot_session_set_snapshot_name.

The patch adds the code for serializing the action and deserializing it
on the sessiond side, but not the code for executing it.

Change-Id: I2b76680d44bf69eb705f2a238fffef2519b82534
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>