[codebuild] implement AWS CodeBuild parsing support

[jimsheldon]

This is just for parsing the CodeBuild yaml spec, we will handle conversion to Harness yaml in a different issue.

Build spec for AWS CodeBuild is here https://docs.aws.amazon.com/codebuild/latest/userguide/build-spec-ref.html

Before we start, convert/codepipeline should be renamed to convert/codebuild. CodePipeline is a separate AWS product, CodeBuild is their CI product (that can be used in CodePipeline).