Define and implement the objects exchange by the issuer and holder during credential issuance

[swcurran]

In AnonCreds v1, there are credential-offer and a credential-request objects that must be exchanged in order to issue a credential.

Define the purpose of the objects and the objects themselves, including any correctness proofs that must be included in the message. At minimum, the process must define how the holder can provide a blinded link secret to bind the credential to the holder. Based on the credential schema types, there may be additional information to flow between the issuer and holder.