Resource pre-planned private data

[jbardin]

Add ability to forward PlannedPrivate data between plans

Terraform providers sometimes want to be able to plan a computed field which may not be deterministically generated. For example, a random GUID could be generated during plan, making the overall Terraform plan more precise, but there is currently no way to carry that planned value forward to apply.

The problem is that there are always two entirely independent plans for every resource change. Terraform compares these two plans for consistency, so a provider may not return a different computed value, but there is no way for the provider to see what the prior planned value was. This means that a planned value which is not reproducible from the given inputs is not plan-able at all.

This PR allows Terraform to take the private data from the first plan, which is usually discarded, and send it along in the second plan request. The provider can then use that data to recreate any previously planned values, either because they are stored directly, or contain enough seed data to recreate the same result.

We require a new client capability flag for this as well. The server needs to know whether it can rely on the planned private data being returned before it can create a plan depending on that behaviour, so the client must advertise that it intends to do so n the initial request.

[austinvalle]

Very cool! I went to try out the changes with the random_string resource and saw that there was just one place missing mapping logic. You can probably guess where, but here's a patch: https://gist.github.com/austinvalle/ddcfc84878bb2ec4f00b0ce067c0ee34

With those changes I was able to make the enhancements successfully 👍🏻, see hashicorp/terraform-provider-random#745