Support project-wide default for mock_outputs_allowed_terraform_commands in dependency blocks #6257

Describe the enhancement

Add a way to set a project-wide default value for mock_outputs_allowed_terraform_commands so it does not need to be repeated on every dependency block.

Today, every dependency block that uses mock_outputs must also explicitly declare which commands are allowed to use those mocks. In a large repo this setting is almost always identical across every unit, resulting in significant boilerplate that is easy to forget and tedious to keep consistent.

dependency "vpc" {
  config_path = "../vpc"
  mock_outputs_allowed_terraform_commands = ["plan", "validate"]
  mock_outputs = { vpc_id = "mock-vpc-id" }
}

Additional context

Implementation suggestion: A dependency_defaults (I'm not married to the name) block in the root HCL file that is merged into every dependency block. Individual blocks can still override the default.

# root.hcl
dependency_defaults {
  mock_outputs_allowed_terraform_commands = ["plan", "validate"]
}

Implications of the feature: The per-block mock_outputs_allowed_terraform_commands value can take precedence, therefore this would be a non-breaking change.

Alternatives considered:

• Without any changes to terragrunt, use read_terragrunt_config to pull in a shared snippet. This solves the consistency part, but it still requires each dependency block to reference it.
• Similarly, expose the root.hcl locals in which you've defined e.g. mock_outputs = ["plan", "validate"].
• CLI argument / environment variable that disables mock outputs, e.g. terragrunt apply --all --no-mock-outputs.

PoC (Proof of Concept)

No

RFC Not Needed

• I have evaluated the complexity of this enhancement, and I believe it does not require an RFC.