liblttng-ctl: add facilities for lttng_snapshot_output object

liblttng-ctl: add facilities for lttng_snapshot_output object

Internal:
  is_equal, serialize, validate, from_buffer.

Public:
  set_local_path, set_network_url set_network_urls

These APIs are used by the upcoming "snapshot session" action
used to trigger a snapshot on a given condition.

The internal API is added to transmit a snapshot output as part of an
action while the public API is added to clean-up the current
snapshot_output API which will be used by the client to create the
snapshot_session actions.

For instance, with set_local_path, it is no longer necessary to create a
local snapshot output by calling lttng_snapshot_output_set_ctrl_url
with a "file://" protocol.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I00f9521faf9f66890ad6ea9a05ad7f6468f805f8