Add configurable endpoint for S3

[mmalenic]

The AWS SDK allows setting an endpoint for the S3 service - which is useful for running it locally or with MinIO. This value cannot be automatically set from environment variables using aws_config::load_from_env(), although this is an open feature request.

An Option should be added to S3Storage which allows setting an AWS endpoint, that can be passed down to AWSS3Storage and set with something like the S3 config builder set_endpoint_url function.

[brainstorm]

Implemented in #171