feat(ecs): support assign_public_ip in vpc_config for public Fargate clusters

[Will Graham (wlggraham)]

Summary

• Adds assign_public_ip boolean field to the vpcConfig struct in the ECS plugin
• When true, tasks are launched with AssignPublicIp: ENABLED; defaults to DISABLED (no behavior change for existing clusters)
• Enables running Fargate tasks in public subnets without a NAT gateway

Test plan

• Verify existing private-subnet clusters (e.g. ecs-fargate-0.0.1) still work with no config change
• Deploy a test job against the new ecs-fargate-public-0.0.1 cluster in heimdall-config and confirm the task gets a public IP and can pull the ECR image

🤖 Generated with Claude Code

Test Results:

• Successfully ran an ECS Fargate job in a public subnet using the public cluster configuration

Sample ECS Fargate Cluster Config:

- name: ecs-fargate-public-0.0.1
    status: active
    version: 0.0.1
    description: AWS Fargate cluster using public subnets with public IPs (no NAT required)
    tags:
      - type:fargate
      - type:fargate-public
    context:
      max_cpu: 4096
      max_memory: 8192
      max_task_count: 30
      execution_role_arn: arn:aws:iam::myexecutionrole
      task_role_arn: arn:aws:iam::myrole
      cluster_name: prod-data-platform-1
      launch_type: FARGATE
      vpc_config:
        subnets:
          - subnet-id-1
          - subnet-id-2
          - subnet-id-3
          - subnet-id-4
        security_groups:
          - sg-0
        assign_public_ip: true