Provide support for new Terragrunt Stacks commands

[brakf]

Describe the solution you'd like
Some new concepts, syntax & commands surrounding Terragrunt Stacks are currently being finalized:
gruntwork-io/terragrunt#3313

I'd highly appreciate first-level support for the Terragrunt Stacks CLI (terragrunt stack generate/init/apply/...).

possible examples functions in the terraform package:

• TgStackGenerate
• TgStackInit
• TgStackApply
• TgStackOutput
• etc.

Describe alternatives you've considered
Using function RunTerraformCommand as i have seen in the official example: https://github.com/gruntwork-io/terragrunt-infrastructure-catalog-example/blob/main/test/terragrunt/stacks/decoupled_lambda_service_test.go

Additional context
https://terragrunt.gruntwork.io/docs/reference/config-blocks-and-attributes/#stack

The new concepts is a replacement of the existing apply-all command, that is already supported. TgApplyAll ¶