Terragrunt support

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This adds a -terragrunt variant for each supported OpenTofu version, mostly tested the apply job in the full pipeline. Not sure how to do conditional artifact/cache paths, and if the module release exclude is correct.

Image release tags need more work, image_digest is used to pass the suffix, note that in my testing project the generated backend overwrites the TF_ADDRESS setup so state_name is ignored:

include:
  - component: gitlab.com/lei/opentofu/full-pipeline@cb8515a4ca06224187bbdfdcb4f3368f517e147f
    inputs:
      version: cb8515a4ca06224187bbdfdcb4f3368f517e147f
      opentofu_version: 1.8.5

      root_dir: tf/example
      state_name: example

      image_registry_base: registry.gitlab.com/lei/opentofu/internal
      image_name: gitlab-opentofu
      base_os: alpine
      image_digest: -terragrunt

Removing -backend=false from the init command in the validate job seems to fix this error always occurring at the second module:

Error: Backend initialization required, please run "tofu init"

Reason: Initial configuration of the requested backend "http"

If this is the right path I can try to add tests but my shared runner minutes are running low :)

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I guess I should have created the MR from this repository.

The plan job fails for some reason but not always.

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Fixes #50 (closed)