feat: add implicit tfvars support

In order to be able to implement some **genericity** in your code, you should use [Terraform variables](https://www.terraform.io/docs/language/values/variables.html) (in your Terraform files), and environment variables (in your hook scripts):

1. any [predefined GitLab CI variable](https://docs.gitlab.com/ee/ci/variables/#predefined-environment-variables) may be freedly used in your hook scripts or extra options variables (ex: `TF_EXTRA_OPTS: "-var project_name=$CI_PROJECT_NAME"`)

2. you may also use [custom GitLab variables](https://docs.gitlab.com/ee/ci/variables/#custom-cicd-variables) to pass values to your hook script or even directly as Terraform variables [using the right syntax](https://www.terraform.io/docs/cli/config/environment-variables.html#tf_var_name)

1. Use [`tfvars` files](https://www.terraform.io/language/values/variables#variable-definitions-tfvars-files) for non-secret configuration:

    * default `terraform.tfvars[.json]` and `*.auto.tfvars[.json]` files are obviously supported by Terraform,

    * the template also auto-detects any file named `$env.env.tfvars[.json]` (ex: `staging.env.tfvars` for staging environment) and uses it with all related `terraform` commands.

2. any [predefined GitLab CI variable](https://docs.gitlab.com/ee/ci/variables/#predefined-environment-variables) may be freedly used in your hook scripts or extra options variables (ex: `TF_EXTRA_OPTS: "-var project_name=$CI_PROJECT_NAME"`)

3. you may also use [custom GitLab variables](https://docs.gitlab.com/ee/ci/variables/#custom-cicd-variables) to pass values to your hook script or even directly as Terraform variables [using the right syntax](https://www.terraform.io/docs/cli/config/environment-variables.html#tf_var_name)

    (ex: env variable `$TF_VAR_ssh_private_key_file` will be visible as `ssh_private_key_file` Terraform variable in your code)

3. **dynamic variables** provided by the template:

4. **dynamic variables** provided by the template:

    * `environment_type`: the environment type (`review`, `integration`, `staging` or `production`)

    * `environment_name` (set as `$CI_ENVIRONMENT_NAME`): the full environment name (ex: `review/fix-prometheus-configuration`, `integration`, `staging` or `production`)

    * `environment_slug` (set as `$CI_ENVIRONMENT_SLUG`): the _slugified_ environment name (ex: `review-fix-promet-r13zmu`, `integration`, `staging` or `production`)

    # shellcheck disable=SC2154

    log_info "--- \\e[32mplan\\e[0m"

    # implicit tfvars

    env_vars=$(ls -1 "${environment_type}.env.tfvars" 2>/dev/null || ls -1 "${environment_type}.env.tfvars.json" 2>/dev/null || echo "")

    if [[ -f "$env_vars" ]]

    then

      log_info "--- environment-specific tfvars file (\\e[33;1m${env_vars}\\e[0m) found: use"

    else

      log_info "--- no environment-specific tfvars file found: ignore"

    fi

    # shellcheck disable=SC2154,SC2086,SC2046

    terraform plan -out "$tf_plan" $(echo "$extra_opts" | envsubst_cli) $(echo "$opts" | envsubst_cli)

    terraform plan ${env_vars:+-var-file=${env_vars}} -out "$tf_plan" $(echo "$extra_opts" | envsubst_cli) $(echo "$opts" | envsubst_cli)

    # then generate GitLab TF report

    if ! command -v jq > /dev/null

      log_info "--- \\e[32mpre-apply\\e[0m hook (\\e[33;1m${prescript}\\e[0m) not found: skip"

    fi

    # implicit tfvars

    env_vars=$(ls -1 "${environment_type}.env.tfvars" 2>/dev/null || ls -1 "${environment_type}.env.tfvars.json" 2>/dev/null || echo "")

    if [[ -f "$env_vars" ]]

    then

      log_info "--- environment-specific tfvars file (\\e[33;1m${env_vars}\\e[0m) found: use"

    else

      log_info "--- no environment-specific tfvars file found: ignore"

    fi

    if [[ "$tf_plan" ]]; then

      log_info "--- applying upstream plan: \\e[33;1m${tf_plan}\\e[0m"

      terraform apply -auto-approve "$tf_plan"

      terraform apply -auto-approve ${env_vars:+-var-file=${env_vars}} "$tf_plan"

    else

      log_info "--- auto plan + apply"

      # shellcheck disable=SC2154,SC2086,SC2046

      terraform apply -auto-approve $(echo "$extra_opts" | envsubst_cli) $(echo "$opts" | envsubst_cli)

      terraform apply -auto-approve ${env_vars:+-var-file=${env_vars}} $(echo "$extra_opts" | envsubst_cli) $(echo "$opts" | envsubst_cli)

    fi

    # maybe execute post apply script

      log_info "--- \\e[32mpre-destroy\\e[0m hook (\\e[33;1m${prescript}\\e[0m) not found: skip"

    fi

    # implicit tfvars

    env_vars=$(ls -1 "${environment_type}.env.tfvars" 2>/dev/null || ls -1 "${environment_type}.env.tfvars.json" 2>/dev/null || echo "")

    if [[ -f "$env_vars" ]]

    then

      log_info "--- environment-specific tfvars file (\\e[33;1m${env_vars}\\e[0m) found: use"

    else

      log_info "--- no environment-specific tfvars file found: ignore"

    fi

    # shellcheck disable=SC2154,SC2086,SC2046

    terraform destroy -auto-approve $(echo "$extra_opts" | envsubst_cli) $(echo "$opts" | envsubst_cli)

    terraform destroy -auto-approve ${env_vars:+-var-file=${env_vars}} $(echo "$extra_opts" | envsubst_cli) $(echo "$opts" | envsubst_cli)

    # remove gitlab-managed tf state

    if [[ "$TF_GITLAB_BACKEND_DISABLED" != "true" ]]