Define and implement the Credential Schema object -- the basis for the claims and encoding of a credential

[swcurran]

The existing schema object should be retained in AnonCreds v2, but should contain the additional type information needed to enable appropriate encoding of the data to enable swapable signature schemes and additional ZKP capabilities.

Note that in the current implementation, there is both an object that is a repository of schema types (extended information), and a credential schema object that references the specific types of the claims for one schema. We currently expect that those two objects will be collapsed in just the schema (same as AnonCreds v1), but that object should have the types and validators for each of the claims that will be in the verifiable credential issued.