Design document for Crossplane to use terraform in the generation of providers

[kasey]

Part of #262

What problem are you facing?

Terraform providers contain data models and business logic that we can use to interface with cloud providers in a consistent way. However, Terraform's operational and data model differs from Crossplane in significant ways, which an integration needs to take into account. A one-pager will help drive discussion about how we map between the two systems and communicate those decisions long-term.

How could Crossplane help solve your problem?

Crossplane could represent Terraform provider resources as CRDs, and interface with Terraform provider plugins using the terraform-plugin-sdk grpc protocol.

Design Doc: #1677

[kasey]

Doc is in review here https://github.com/crossplane/crossplane/pull/1677/files