feat: add multi-cloud Terragrunt drift detection workflow

[kalyani-cd]

Summary

This PR adds a reusable Terragrunt drift detection workflow with support for AWS, Azure, and GCP.

Changes

• Added reusable tf-terragrunt-drift.yml workflow.
• Added multi-cloud provider support:
  • AWS
  • Azure
  • GCP
• Added provider-specific authentication.
• Added Terraform and Terragrunt version inputs.
• Added Terragrunt plan-based drift detection.
• Added drift and error unit parsing.
• Added workflow outputs:
  • status
  • drift_count
  • error_count
• Added GitHub Actions step summary for drifted and errored units.
• Added Slack notifications for drift and error states.
• Added documentation and usage examples for AWS, Azure, and GCP.
• Ensured GCP-specific authentication and gcloud setup only run when provider: gcp.

Supported Statuses

• clean — No infrastructure drift detected.
• drift — Infrastructure drift detected.
• error — Terragrunt/Terraform execution failed.

Validation

The workflow uses a read-only Terragrunt plan:

terragrunt run --all plan -- \
  -lock=false \
  -input=false \
  -no-color \
  -detailed-exitcode