retry_in parameter for deferred credentials

[bellebaum]

It seems unclear to me how often the client should retry to retrieve a token that could not yet be issued.
Maybe an option for the server to specify when it expects to be able to deliver the requested credential would be helpful.
This parameter (let's call it retry_in) would (optionally or mandatory) be returned with the acceptance_token at the credential endpoint, and at the deferred credential endpoint whenever the credential could not yet be delivered.

If applicable, please also tell me if the issue is best discussed at the OIDC WG, as I had to leave early during the call yesterday.