Adding "schema" property to the "descriptor" when adding credentials

[YassirSellami]

Currently a credential can be verified by implementing a CredentialEnvelopeVerifier for the right format. However this verifier cheks only the validity and the veracity of the format and proof.
So to be able to do validation on the actual content of the credential another method is needed, in that case the property "schema" from the Dencetralized Web Node can be added and used the same way as the data format for credentials.
For exemple this could be used to validate that the content of a self description credential is in the correct format, or that all relevant properties of a service offering credential are available