Add did:keri work item

[peacekeeper]

As discussed in meetings:

• https://github.com/decentralized-identity/identifiers-discovery/blob/main/agenda.md#meeting---22-november-2021---1400-et-recording
• https://github.com/decentralized-identity/identifiers-discovery/blob/main/agenda.md#meeting---06-december-2021---1400-et-recording

@Joachim16 @Exulansis @chunningham

[peacekeeper]

Following some discussions in the ID WG and various community events, this work item will not go ahead for now. Closing the PR.