Issue Credential: initiate with issue message

[dbluhm]

Issue Credential as a protocol was formulated with support for credential formats like AnonCreds that require exchange of key cryptographic primitives prior to issuance. This resulted in the offer-request-issue-ack and proposal-offer-request-issue-ack flows we have today. However, not all credential formats have these requirements. LDP-VC credentials can skip straight to the issuing a credential in many circumstances. The protocol as specified right now requires that all issuer-initiated flows start from the offer message, despite the differing requirements between credential formats.

This has been discussed on the Aries WG call and the resolution was that it's fine for credential formats that don't require prior steps to begin with the issue message.

We should update the protocol to reflect this resolution.

Implications to discuss: how does this affect the version of the protocol?

cc @TelegramSam