FEP-8b32: Object Integrity Proofs

@helge Are you planning to move this file to fediverse-features?

I want to add test vectors for FEP-8b32, but I am not sure how to do it. Gherkin syntax looks nice. It is human readable, and it is definitely better than plain JSON files. However, I think it would be better to put test vectors alongside the FEP, and not in a separate repo.