Part 1 of changes to use REUSE software for managing licenses and copyrights

[jafingerhut]

Move all license files into a new top level LICENSES directory. Add <filename>.license files with copyright and license information for file formats that do not support comments, such as the font files for which such files are added in this commit.

[jafingerhut]

This is part 1 of a set of similar changes intended to use the REUSE open source software for managing copyrights and licenses of every file in p4lang repositories.

Properties:

• An open source reuse lint command can be used in CI to validate that every file in the repository has copyright and license information. This PR by itself does not get to that point, but it is a step in that direction.
• Files that support comments have their copyright and license information added into comments inside the file, with specific supported syntax like "SPDX-License-Identifier: Apache-2.0" to specify the license the file is released under.
• Files that do not support comments inside of them, e.g. the binary TrueType .ttf font files in the p4runtime repo, have a separate file named <filename>.license added that contains the copyright and license information.
• All license files are stored in a top level directory named LICENSES, one license per file, with standard names for common licenses tracked by SPDX at https://spdx.org/licenses/
• There is a reuse annotate command that can be used to add this information for one file, or many files at once, similar to Google's addlicense command in its features. The Bash script tools/reuse-annotate-font-files.sh in this PR demonstrates how this was done for the font files in the repository.