Proper support for W3C VC

[salboaie]

• compare W3C VC standard with current OpenDSU JWT based VCs
• check the JWT standard (replace base58 with base64) encoding
• check the format of the signature
• allow APIs to add custom/extended claims in credentials
• allow linking of external credentials (instead of embedding them) : put an extended claim as a hash of an external document
• create a test with a ValidationStrategy (as example on how these credentials should be used)