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

See merge request components/opentofu!190

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

Rename `TF_GITLAB_SOURCED` variable to `GITLAB_TOFU_SOURCED`

See merge request components/opentofu!188

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

Remove `TF_FF_AUTO_URLENCODE_STATE_NAME` feature flag

See merge request components/opentofu!187

This feature flag is now always enabled.

Changelog: removed

Remove `TF_FF_AUTO_APPROVE_APPLY` feature flag

See merge request components/opentofu!186

Rename internal `OPENTOFU_COMPONENT_` variables to `GITLAB_TOFU_`

See merge request components/opentofu!185

This feature flag is now always enabled.

Changelog: removed

Support env variable to force sourcing `gitlab-tofu`

See merge request components/opentofu!184

Changelog: development

Changelog: added

Support custom runner tag for image build job

Closes #73

See merge request components/opentofu!183

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

Changelog: development

Support OpenTofu 1.9.0-alpha1

See merge request components/opentofu!182

Changelog: added

Update debian Docker tag to v12.8

See merge request components/opentofu!180

Fix manual destroy jobs

Closes #82

See merge request components/opentofu!179

Closes https://gitlab.com/components/opentofu/-/issues/82

Changelog: fixed

Fix plan json generation if `warning_on_non_empty_plan` is `true` and plan is non-empty

Closes #81

See merge request components/opentofu!178

Closes https://gitlab.com/components/opentofu/-/issues/81

Changelog: fixed

Remove `auto_apply` and `auto_destroy` inputs

Closes #80

See merge request components/opentofu!177

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

Remove duplicate image digest from `apply` job

See merge request components/opentofu!176

Changelog: fixed

Rename `warning_on_none_empty_plan` to `warning_on_non_empty_plan`

See merge request components/opentofu!173

Document glab and provider login

See merge request components/opentofu!174

Changelog: changed