Consolidate validation logic

[mnzaki]

Description

Currently in two places: utils/validation and identityWallet.validateJWT. Perhaps it needs a "Validator" class/interface of some sort? or perhaps just keep them as utils?

Note that the validation logic paths are the only ones that needs to be able to resolve non-jolo DIDs and so require a resolver function. Containing that requirement into a new class/interface can be helpful.

Part of #355

TODO

• decide about class vs utils and refactor accordingly
• move out validateJWT logic and deprecate its usage
• what about SoftwareKeyProvider.verifyDigestable ?