SLSA Playground - demo for how SLSA works in practice

[MarkLodato]

I put together a quick demo of how SLSA can be integrated into a package registry in order to prevent attacks. I'm hoping that it makes the concepts more concrete. The code itself is hacky, but hopefully it's easy enough to understand. I'm happy to field any questions or suggestions, but be aware I'm about to take 5 weeks of leave (until the week of April 8).

Link: https://github.com/MarkLodato/slsa-playground

If this is useful, we can move it to the slsa-framework repository, expand it where needed, and add it to our official documentation.

Happy to hear feedback, positive or negative!