actions: introduce function typedef for creating actions from buffer

The only existing action type, LTTNG_ACTION_TYPE_NOTIFY, does not
require deserializing additional data (on top of the action type field),
so lttng_action_create_from_buffer handles it in a trivial way.
Upcoming patches will introduce new action types which will need to
deserialize some additional data.  This patch prepares
lttng_action_create_from_buffer for that by making it call an
action-specific function for deserializing this additional data.  The
changes are inspired by what lttng_condition_create_from_buffer does.

No functional changes intended.

Change-Id: I469a67b744aa2cf7a45d7d970f1bbee6a994a2a9
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>