Generate and validate SRI hashes

[decentralgabe]

          We will also need to validate that a VC that's setting the `credentialSchema.digestSRI` property, has a value which matches against the locally calculated digestSRI. This typically involves dereferencing `credentialSchema.id`, and then following the spec of SubResource Integrity. 

I would prefer that we do that in this PR, so that the same mechanism is used for every VC (instead of special casing VCs who's type property is JsonSchemaCredential). That said, I'd understand leaving that for another PR. Up to you.

Originally posted by @andresuribe87 in #455 (review)