Migrate to programatic aws-cdk

[sladg]

Once ready, we want to remove existing exec() child process implementation of CDK.

AWS announced @aws-cdk/cli-lib-alpha which adds programatic access, however, it's very simple at current stage and flags, env vars, etc. are not supported for now.

Once stable, use it.

See more: https://docs.aws.amazon.com/cdk/api/v2/docs/cli-lib-alpha-readme.html#deploy