Ability to specify S3Key for lambda functions

[aaronbrashears]

I would like to use aws cdk to deploy this tool so that teams can have this as part of their cdk application rather than have piecemeal cdk and cloudformation templates. The easiest way to deploy custom lambdas with the cdk is to wrap them in an aws-s3-assets.Asset construct. That construct uploads to a generated key and not in the root of a particular bucket. I would like the ability to specify the key of the lambda files so that we can do this. It could be generally useful for other deploy systems.

I can make a pull request for this if there is interest.