Misc: Add pre-commit check for SPDX License-Identifier and FileCopyrightText

Misc: Add pre-commit check for SPDX License-Identifier and FileCopyrightText

Observed issue
==============

There are frequently small typos or slightly deviant SPDX tags set on files.

Cause
=====

Mostly me.

Solution
========

Add a pre-commit check with the following allowed tags:

 * SPDX-License-Identifier
 * SPDX-FileCopyrightText
 * SPDX-URL

Known drawbacks
===============

This is not a full SPDX validation, and does not validate the license
identifier values.

Change-Id: Iabd78ae14ef143ab4a956e0c5c31c1490b43aeda
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>