lib: add bt_version_get_extra_{name,description,patch_names}

lib: add bt_version_get_extra_{name,description,patch_names}

Those new functions return extra information about the library's version
for custom builds (see `version/README.adoc`).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I52abca8235826d1e336584285e925147895b13f4
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2838
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>