fix(codepipeline-actions): set service as backing resource for EcsDeployAction

[strax]

Adds cross-account deployment support to EcsDeployAction CodePipeline action by setting the ActionProperties.resource property introduced in #3012 to the target ECS service.

Pull Request Checklist

• Testing
  • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
  • CLI change?: coordinate update of integration tests with team
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
  • Design: For significant features, design document added to design folder
• Title and Description
  • Change type: title prefixed with fix, feat and module name in parens, which will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)

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[skinny85]

Thanks for the subtle PR @strax !

Note that I haven't explicitly tested whether cross-account support works for ECS services (yet) - it might be that we will need to create some Roles inside the service, for example, with a lazy-assigned physical name. Let us know what you find out!

Thanks,
Adam