Add an S3 file backend

This also involved adding debusine-admin asset create and debusine-admin file_store create --provider-account, and a few miscellaneous bits of tidying up. The file backend itself is only a few hundred lines of code including tests, so it should be pretty easy to add more like this for other providers.

I decided to just make python3-boto3 and python3-botocore hard dependencies for now as they aren't all that heavyweight, but if we end up with too many such libraries we might want to consider splitting extra backends into add-on packages. For now, that seemed like unnecessary complexity.

Fixes: #685 (closed)