(synth): Toolkit should cleanup old assets in the cdk.out directory

Each time I run a CDK deploy I get a new asset in the asset's directory, and they seem to accumulate forever. Each asset folder is around 100 MB for me, so this quickly adds up to many GB of data. Here is a screenshot of it accumulating assets again after the last time I cleaned it out manually.


Ideally I would like a CDK configuration that would cause CDK to automatically garbage collect older asset files it no longer needs so I don't have to do it manually.