src.ctf.fs: make ctf_fs_ds_index_create return a unique_ptr

src.ctf.fs: make ctf_fs_ds_index_create return a unique_ptr

Introduce ctf_fs_ds_index::UP, a unique_ptr with a deleter that calls
ctf_fs_ds_index_destroy.  Make ctf_fs_ds_index_create (and the
other functions creating in index in data-stream-file.cpp) return it.

Change-Id: Id04124aeaff3384e566a43fbbd6c1fedd2bf9bc3
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8251
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>