Update Verifier's verify method to return Result type instead of boolean

[Gozala]

Currently Verifier interface defines verify method as follows:

interface Verifier<ID extends DID = DID, Alg extends SigAlg = SigAlg> {
  verify<T>(payload: ByteView<T>, signature: Signature<T, Alg>): Await<boolean>
  // ...
}

This really problematic because it makes it impossible to communicate why verification has failed. In some cases it may be because signature is invalid, or because signature algorithm does not match the algorithm of the verifier. Or perhaps verifier needs to load key on demand but fail to do so.

Result type is much better fit as it can communicate reason why verification has failed.