Resource references not being populated properly

[morgante]

When resources are cross-referenced, the dependent field is computed (not known until after apply). Currently, we omit computed fields entirely but this breaks validations (for example).

Instead, we should fill in computed fields with a sensible default/fake value which can be used to pass validations.

[morgante]

I took an initial stab at this in #143 which injects _computed_ for all computed properties but this ends up polluting output quite a bit. It's also not type-aware.

Overall, I think we want to take a more fundamental look at how generation happens before trying to fix this. In particular, I think our customer TerraformResourceData approach seems potentially misguided in the 0.12 world and we should look at integrating https://github.com/hashicorp/terraform-plugin-sdk more.

[marktru]

Internal issue: b/211887747