ctf: introduce dry-run mode for `bt_msg_iter`

ctf: introduce dry-run mode for `bt_msg_iter`

Dry-run mode can be used when component classes need to decode CTF
packets but do not want the `bt_msg_iter` to create and use the
Babeltrace library objects (bt_field, bt_message_*, etc.).

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I336ec4c893835e7b1d6f32262841793f7543aa06
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1417
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>