Rework Terraform plugin for PR flow

[arm4b]

Per multiple feedback, we should rework the Terraform plugin flow to mimic the collaborative Terraform Cloud model which fits better production use of Terraform in the wild.

The Terraform plugin should integrate with Github and Gitlab to be able to work nicely with the PR/review/approve flow where the terraform plan diff is included in the PR comment via bot which the reviewer can approve.
Additionally, we should make sure to include sufficient docs and examples to guide users with the best practice.

Resources

• https://cloud.hashicorp.com/products/terraform
• Example: https://github.com/marketplace/actions/terraform-pr-commenter
• Screenshots: https://github.com/marketplace/actions/terraform-pr-commenter#screenshots