Make the project reuse compliant

This merge request adds all the usual suspects:

• SPDX headers
• LICENSES folder
• .reuse folder
• pre-commit hook to keep it clean

.reuse/dep5

+Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
+Upstream-Name: exposed
+Upstream-Contact: Andre Anjos <andre.anjos@idiap.ch>
+Source: https://gitlab.idiap.ch/bob/exposed
+
+Files: tests/data/*
+Copyright: Copyright © 2022 Idiap Research Institute <contact@idiap.ch>
+License: BSD-3-Clause