[S2S]: Ratify Credential Provisioning flow

[GarethCOliver]

Issue tracking consensus on the credential provisioning flow. Text is currently as follows:

• CredentialInstanceIds are returned from the 'credential/provision' endpoint after successful verification
• 'credential/get' is called per CredentialInstanceId, with an array of 'presentationKeys' and relevant proofs.
• The Wallet Server performs the fan out for multiple CredentialInstanceIds
• Asynchronous gets are handled by returning PENDING and recalling the endpoint later
• Credential Metadata can be returned from the 'get' endpoint OR retrieved independently from the 'credential/metadata' endpoint.