actions: introduce start session action

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

A start session action is created using the
lttng_action_start_session_create function.  It is mandatory to set a
session name using lttng_action_start_session_set_session_name before
using the action in a trigger.

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: I90598e25a461ccabcf7dc327aaa73b3d35e203af
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>