feat(aws-ecs): Add scaling ECS service on ALB Request Count

[BDQ]

Adds a scaleOnRequestCount inline with scaleOnCpuUtilization and scaleOnMemoryUtilization

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Pull Request Checklist

• Testing
  • Unit test added
  • CLI change?: manually run integration tests and paste output as a PR comment
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
• Title and Description
  • Change type: title prefixed with fix, feat 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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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.

[BDQ]

@rix0rrr that formatting commit seems to have angered the linter:

@aws-cdk/aws-ecs: > @aws-cdk/aws-ecs@0.22.0 build /home/travis/build/awslabs/aws-cdk/packages/@aws-cdk/aws-ecs
@aws-cdk/aws-ecs: > cdk-build
@aws-cdk/aws-ecs: ERROR: /home/travis/build/awslabs/aws-cdk/packages/@aws-cdk/aws-ecs/lib/base/scalable-task-count.ts:50:1 - trailing whitespace
@aws-cdk/aws-ecs: Error: tslint --project . exited with error code 2

[rix0rrr]

Oh that's fun. The GitHub editor will always insert a trailing newline to the file, and our linter always complains about it.