feat: add drift-detection workflow

[CalvinRodo]

This pull request introduces a new GitHub Actions workflow for automated drift detection in Terraform or OpenTofu-managed infrastructure. The workflow is designed to be reusable, supports both Terraform and OpenTofu, and posts detailed drift results to Slack for visibility.

Key changes:

Drift Detection Workflow:

• Adds a new workflow file .github/workflows/drift-detection.yml that can be triggered via workflow_call and accepts configurable inputs such as tool versions, target directory, and Slack webhook.
• Supports both Terraform and OpenTofu for drift detection, automatically selecting the tool based on provided input parameters.
• Runs drift detection using the cds-snc/terraform-plan action and captures structured drift output for reporting.

Slack Integration:

• Posts a structured summary of drift results (created, updated, deleted resources) to a specified Slack channel using the slackapi/slack-github-action, improving visibility and response time for infrastructure changes.