is there an amazonica/s3 builtin for "rsyncing an directory to s3" ?

I see the put-object get-object files, but I want to do it to entire dir not a single object

in particular, I'm looking for the amazonica equiv of 'aws sync'

yes

https://docs.aws.amazon.com/AWSJavaSDK/latest/javadoc/com/amazonaws/services/s3/transfer/TransferManager.html

there is an .uploadDirectory() and .uploadFileList()

it's not quite aws sync since it'll overwrite stuff that is already there. if you need aws sync I suggest just shelling out to it. I do that for deployments and I've heard of several others doing the same thing.