Refs https://gitlab.com/components/opentofu/-/issues/83

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Support running pipeline templates directly in child pipeline with `trigger_in_child_pipeline` input

This change set allows a user of this component to directly execute a
pipeline template in its own child pipeline. This is useful to run
dedicated child pipeline for multiple environments.

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This change set renames the `TF_GITLAB_SOURCED` environment variable to
`GITLAB_TOFU_SOURCED` and document the existence of the variable
accordingly in the README.
The reason for renaming it is that the `TF_` variable prefix is "owned"
by OpenTofu / Terraform and not GitLab Tofu. Therefore, we make an
effort to rename these variables to have the `GITLAB_TOFU_` prefix
instead.

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Closes https://gitlab.com/components/opentofu/-/issues/73

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This change sets removes support `auto_apply` and `auto_destroy` inputs
across the templates.
With the introduction of the `rules` inputs, an auto apply or auto
destroy behavior can easily implemented using it directly.

We can't easily keep the `auto_apply` and `auto_destroy` inputs because
having input references in input default values is not yet supported.
See https://gitlab.com/gitlab-org/gitlab/-/issues/438722.

I've left the input definitions commented out for now with the
possibility to easily bring it back if the issue is resolved.

Closes https://gitlab.com/components/opentofu/-/issues/80

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Closes https://gitlab.com/components/opentofu/-/issues/70

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Closes https://gitlab.com/components/opentofu/-/issues/23