src.ctf.fs: make ctf_fs_trace::ds_file_groups a vector of ctf_fs_ds_file_group::UP

src.ctf.fs: make ctf_fs_trace::ds_file_groups a vector of ctf_fs_ds_file_group::UP

Replace the GPtrArray with an std::vector of unique_ptr, simplifying
memory management.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I47a6b60a35b6d776a7234af1950e460da8c1e1f4
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8247
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12285