Add GitHub Actions workflow to plan tofu changes on PRs

[gonzolino]

Add a new workflow to plan tofu changes in the automation directory before they get merged.

This PR builds on the changes of #53.

[github-actions]

Plan: 🔄 Changes pending

Show plan

OpenTofu used the selected providers to generate the following execution
plan. Resource actions are indicated with the following symbols:
  ~ update in-place (current -> planned)

OpenTofu will perform the following actions:

  # github_organization_ruleset.default_release will be updated in-place
  ~ resource "github_organization_ruleset" "default_release" {
        id          = "18719553"
        name        = "Protect default and release branches"
        # (5 unchanged attributes hidden)

      ~ rules {
            # (6 unchanged attributes hidden)

          ~ pull_request {
              ~ dismiss_stale_reviews_on_push     = false -> true
                # (5 unchanged attributes hidden)
            }

            # (3 unchanged blocks hidden)
        }

        # (1 unchanged block hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.

Workflow: 35621450577