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.

Changelog: added

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.

Changelog: changed

This feature flag is now always enabled.

Changelog: removed

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

Changelog: removed

Changelog: changed

Changelog: added

Changelog: added

Changelog: fixed

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

Changelog: other

This change set implements auto url-encoding for the state name when
it's used as a URL.
This allows to seamlessly define a state name like `production/aws`.

The feature is behind a feature flag that can be disabled.
You can define the `TF_FF_AUTO_URLENCODE_STATE_NAME` variable as `false`
in your pipeline configuration to disable this behavior.

Changelog: added

This change set removes the machinery to backport the CI/CD component to
CI/CD templates.

Closes https://gitlab.com/components/opentofu/-/issues/43

This change set mocks the CI_SERVER_HOST predefined environment variable
with a deterministic value in the unit tests.
This is necessary so that the unit tests run on self-managed.

Changelog: fixed

$CI_REGISTRY <- registry.gitlab.com
$CI_SERVER_FQDN <- gitlab.com
$CI_PROJECT_PATH <- components/opentofu

Closes: https://gitlab.com/components/opentofu/-/issues/36

git grep -l '\s\+$' | xargs sed -e 's/\s\+$//' -i