[#137] Add AutoScaling feature to ECS cluster

[longnd]

• Close Add Auto Scaling to ECS cluster #137

What happened 👀

• Modify the ECS module to enable the AutoScaling feature
• Use Terraform locals instead of data to define ECS task execution role to be consistent with the other policies
• Add Liam as the CODEOWNER
• Fix some incorrect variables & locals reference

Insight 📝

This PR follows the implementation in recent projects, i.e. Flexi Payroll & easyHotel

• https://github.com/onward-venture/ewa-payroll-infra/pull/24 (thanks to Hoang)
• https://github.com/easyhoteluk/aws-infrastructure/pull/128 (thanks to Terone)

with some slight modification

• Add cooldown period
• Allow external changes without Terraform plan to the desired_count of ECS service as it can be changed by Autoscaling

  lifecycle {
    ignore_changes = [desired_count]
  }

Proof Of Work 📹

• The tests pass
• Initializing the generated project did not face any issue