Support specifying a custom endpont_url for S3

[ikreymer]

Is your feature request related to a problem? Please describe.

boto3 allows custom endpoint_urls, but they can not be set via an environment variable.

This would allow using s3:// paths with other AWS-compatible S3 deployments.

Describe the solution you'd like

Provide a way to set the boto3 client endpoint_url from either an env var or a config.yaml option.