feat: support tools such as OpenTofu instead of Terraform

[suzuki-shunsuke]

Close #1203
Close #1551

Caution

We don't usually use OpenTofu and Terragrunt, so we haven't done only simple verification.

Overview

This pull request enables us to execute tools compatible with Terraform instead of Terraform.
tfaction executes Terraform commands such as terraform init, fmt, validate, plan, apply, and so on.
This pull request enables us to replace Terraform with other tools compatible with Terraform.
You can use tools such as OpenTofu and Terragrunt instead of Terraform.

How

You can specify a tool by the setting terraform_command in tfaction-root.yaml and tfaction.yaml.

tfaction-root.yaml

terraform_command: tofu # terragrunt
target_groups:
  - working_directory: aws/
    terraform_command: tofu # terragrunt

tfaction.yaml

terraform_command: tofu # terragrunt

Then the given command is executed instead of terraform.
For example, if terraform_command is tofu, commands such as tofu init, fmt, validate, plan, apply are executed instead of terraform.

💡 Combine OpenTofu and Terragrunt

You can also combine OpenTofu and Terragrunt.

1. Set terraform_command to terragrunt
2. Set the environment variable TERRAGRUNT_TFPATH to tofu

💡 Validate terraform_command

You can validate terraform_command in GitHub Actions Workflows.

      - uses: suzuki-shunsuke/tfaction/get-target-config@v1.2.0
        id: target-config

      - run: |
          echo "::error:: terraform_command is invalid"
          exit 1
        if: |
          ! contains(fromJSON('["terraform", "terragrunt", "tofu"]'), steps.target-config.outputs.terraform_command)