Terms: add issuer service and verifier service #411
Description
Issuer service: An entity that performs the act of creating and delivering a verifiable digital credential on behalf of the issuer. In many cases, the issuer and issuer service are the same entity or component.
Verifier service: An entity that performs the act of creating a presentation request, making a presentation request, and performing cryptographic validation of a verifiable presentation on behalf of the verifier. In many cases, the verifier and verifier service are the same entity or component.