src.ctf.fs: make ctf_fs_trace::metadata a unique_ptr

src.ctf.fs: make ctf_fs_trace::metadata a unique_ptr

Introduce ctf_fs_metadata::UP and change the ctf_fs_trace::metadata
field to be of that type.

Change-Id: I0221ef7786cc8fb4c2e8a9264573ab5c2e5420d6
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8286
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12324
Tested-by: jenkins <jenkins@lttng.org>