(feat) set stack outputs as action outputs

[clareliguori]

Any stack outputs will be set as action outputs

Example:

    - name: Deploy ECS Service with CloudFormation
      id: service-stack
      uses: aws-actions/aws-cloudformation-github-deploy@v1
      with:
        name: github-actions-service
        template: cloudformation-templates/service-fargate-public-subnet-public-lb.yml
        parameter-overrides: >-
          ImageUrl=nginx
    - name: Print service stack outputs
      env:
        OUTPUTS: ${{ toJson(steps.service-stack.outputs) }}
      run: echo "$OUTPUTS"

My CFN template has an output "ServiceURL", so this is the sample output:

{
  "stack-id": "arn:aws:cloudformation:***:***:stack/github-actions-service/53a97cb0-a2dd-11ea-b6d3-0608930970a0",
  "ServiceURL": "http://clare-Publi-E3GRD5LG86GN-138111391.us-east-1.elb.amazonaws.com"
}

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