Fix REUSE licensing/copyright issues in `src`

Fix REUSE licensing/copyright issues in `src`

This patch makes `reuse lint` happy for the files in `src`.

Putting the info into a separate `.license` file for
`src/cpp-common/optional.hpp` to avoid modifying the original file which
comes from an external project.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I073afa1b5de0906f4f9f2a809ef387c05833ca17
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11384
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>