src.ctf.fs: add `force-clock-class-origin-unix-epoch` param

src.ctf.fs: add `force-clock-class-origin-unix-epoch` param

The `force-clock-class-origin-unix-epoch` initialization parameter makes
all the created clock classes have a Unix epoch origin. This is useful
to force the clock classes of multiple traces to be compatible even if
they are not inherently.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: If311081052e9be28ca5782ee684b4ef7bba66176
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2069
Tested-by: jenkins <jenkins@lttng.org>