Feature request: $util functions to generate signed S3 URL's

I currently have an API that manages a lot of files stored in S3, with the file metadata stored in a DBD table. It would be nice if there was a VTL mapping template $util function that could generate a pre-signed URL for an S3 object right in the template. Currently, one has to create a lambda function that will generate these URL's, and thus incur the lambda startup time and billing.
It would seem like a simple thing to add as the generation of the URL doesn't require any network requests, it just needs to generate the signature.

Start with the VTL mapping-template $util functions and the S3 object flow described in the issue. Review how AppSync utilities receive signing inputs and define the expected pre-signed URL behavior; done means the requested URL can be generated in a template without a Lambda function.

Written by the indexing model from the issue text.