We need to retain the prior state (in a form that Terraform Core's modules
runtime can accept) between the plan and apply phases because Terraform
uses it to verify that the provider's "final plan" is consistent with
the initial plan.
We previously tried to do this by reusing the "old" value from the planned
change, but that's not really possible in practice because of the
historical implementation detail that Terraform wants state information
in a JSON format and plan information in MessagePack.
This also contains the beginnings of handling the "discarding" of
unrecognized keys from the state data structures, although we'll probably
need to do more in that area later since this is just the bare minimum.
This is a sketch of the overall structure of the prior state decoder and
the model type it populates.
Before we can complete this we'll need to slightly rework how the apply
phase emits the raw events that this is consuming, and in particular to
change the raw state representation to be JSON-based to match with how
Terraform Core expects to receive it once reloaded. That will follow in
later commits.
