src.ctf.fs: make ctf_fs_ds_index_entry_create return a unique_ptr

src.ctf.fs: make ctf_fs_ds_index_entry_create return a unique_ptr

Introduce ctf_fs_ds_index_entry::UP and make
ctf_fs_ds_index_entry_create return that.  Modify a few functions in
data-stream-file.cpp that deal with this.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I39598d860327cff5af32804f75b68b58e4ee0821
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8253
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12291