[LFX Internship] Management of Terraform state

To some extent, Terraform state is the most essential component for cloud resources provisioned by Terraform Controller. We need to

- Support custom Terraform backend https://github.com/oam-dev/terraform-controller/issues/111
- Backup and restore Terraform state
- Upgrade Terraform state when Terraform binary and Terraform provider got upgraded

Start by reviewing the custom Terraform backend discussion in issue #111 and the Terraform Controller's existing state-management behavior. Define completion around supporting custom backends, backup and restore, and state upgrades when the Terraform binary or provider changes.

Written by the indexing model from the issue text.