Construct a real-world example that contains supply chain metadata

[radu-matei]

So far we've been testing signy with the default "hello world" in-toto project layout, which is not necessarily representative of a real-world scenario.

I'd also like to show how to use delegations for the in-toto root layout key.

cc @squillace, @trishankatdatadog

[radu-matei]

I can't officially assign @squillace, but I unofficially also assign @squillace.

[squillace]

You feel free