Parse "addons" folder to generate template.yml

[efekarakus]

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The ecs-project/{appName}/ can contain an optional addons/ directory with the following files:

addons/
├── params.yml
├── policy.yml
├── {resource}.yml # There can be multiple files per resource
└── outputs.yml

We want to merge all these files into a single CloudFormation template such that the template sections are:

Parameters:
  # Contents from params.yml.
Resources:
  # For every {resource.yml} copy/paste content.
  # Add the IAM ManagedPolicy from policy.yml here.
Outputs:
  # Contents from outputs.yml

Parent story

This is to support: #644