Add support for the AWS Lambda Layers feature, enabling users to define
reusable and shareable packages of runtime code that can be shared
across multiple lambda functions.

BREAKING CHANGE: The lambda.Code class has an additional attribute named
isInline. This breaks all third-party extensions of the lambda.Code class,
maintainers will need to add the missing attribute.

Pull Request Checklist

• Testing

  • Unit test added
  • CLI change?: manually run integration tests and paste output as a PR comment
  • CLI 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)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.