Function verifyProofGraph

Verify the separate proof graph.

The validity result is the conjunction of the validation result for each proof graphs separately.

The following checks are made:

1. There should be exactly one proof value
2. There should be exactly one verification method, which should be a separate resource containing the key (either in JWK or in Multikey)
3. The key's (optional) expiration and revocation dates are checked and compared to the current time which should be "before"
4. The proof's creation date must be before the current time
5. The proof purpose(s) must be set, and the values are either authentication or verification

If any of those errors are found, the validation result is false. The error reports themselves, with some more details, are part of the verification result structure.