Initial implementation

> Fixes [EZP-23018](https://jira.ez.no/browse/EZP-23018)
> Status: ready for review

Implements a FSBackend for aws-s3, using the official SDK.

# Objective of the review
If there are no *mandatory* features that are missing, merge this to master, and release beta1.
If we must delay, we can also list what's missing, merge this to master and release another alpha1.