chore: Add REUSE compliance and AI block

[KhronosWebservices]

• add reuse compliance to the repository
• add an AI Contributing block to the CONTRIBUTING doc
• add an Open Source Apache 2.0 CLA to cover in-bound contributions.

This repository CONTRIBUTING.adoc originally listed only CC-BY-SA-4.0. However, the repository also has Apache-2.0, CC-BY-4.0 and BSD-3-Clause.

As there are typically a lot of questions around the use of wildcards in REUSE.toml, here is a little background:

The wildcard has been put in place solely because REUSE was introduced without first updating every file. A strict configuration would have blocked any commit touching a non-compliant file, which probably would not make the maintainers very happy. The wildcard gives us a compliant baseline while headers are added at the maintainers' pace.

REUSE annotations are not just for exceptions. The REUSE spec fully supports REUSE.toml as the primary mechanism for declaring copyright and licensing. File headers are recommended, not required.

1. Wildcards can be removed once file headers are in place across the repository.
2. Updating headers is the correct thing to do. However, the wildcard is a valid way to use REUSE.toml.
3. Once the wildcard is removed, missing licences will trigger lint failures.