zhsh

A quick note: Smallstep's implementation of ACME client and server already supports the new extension. Smallstep client code that supports both the default and non-default payloads: https://github.com/smallstep/certificates/blob/1b2d999e4607bbe4796dce2a0f0f3c7a29cec463/ca/acmeClient.go#L243-L267

> Would it make sense to add a field to the Challenge type instead of having a whole new method? This works too. If I understand correctly, the change will...

> We could also avoid adding a field entirely, and add an internal check for `Challenge.Type == "hardware-module"`. This would be somewhat opaque, but doesn't require an API change. I...

Thank you for the review and suggestions!

> OK, so it sounds like maybe adding > > ``` > Payload json.Message > ``` > > to Challenge is all we need? This looks like the best path...