From https://freexian-team.pages.debian.net/debusine/reference/devel-blueprints/dynamic-storage.html, and assuming that #682, #683, and #684 have all been completed first: * [x] Implement an Amazon S3 file store, with configuration holding the necessary API keys. `get_url` should be implemented using a [presigned URL](https://docs.aws.amazon.com/AmazonS3/latest/userguide/using-presigned-url.html) with a short expiration time, e.g. using [boto3](https://boto3.amazonaws.com/v1/documentation/api/latest/guide/s3-presigned-urls.html). (!1643) Report time in #539.