ITE-10: Support contextual predicates in layouts

[adityasaky]

This was previously part of #38. I've broken that one down into separate ITEs so it's easier to reason about the changes proposed.

Prototyping here: https://github.com/adityasaky/in-toto-attestation-verifier

[mikhailswift]

I like the distinction of 'transformational' and 'informational' predicates. I think that succinctly captures a lot of what we learned with Witness and the distinctions we found between our attestors. I think this ITE is looking good.

[TomHennen]

This makes total sense to me!

[marcelamelara]

Thanks @adityasaky !

[SantiagoTorres]

I like this, and I'm happy to merge this as draft. I'd still want to explore the security position of sublayouts a bit more on a future iteration (e.g,. when marking as active).

[marcelamelara]

Thanks for the updates @adityasaky . I have a few clarifying comments but really like where this ITE is going.

[marcelamelara]

Adding some more comments to my review.

[marcelamelara]

Thank you for all your work on this @adityasaky !