Support S3 compatible state stores

# Description

It would be great to support other S3 compatible stores as the state store. I am currently unable to get Digital Ocean Spaces to work, as it needs the endpoint to be changed.

# Technical Information

S3LevelDown is currently being used for the `S3StateStore` implementation. Their docs have an [example](https://github.com/loune/s3leveldown#example-with-minio) of how to configure other S3 stores like Min.io. I believe this would require a few more config parameters, notably a custom `endpoint`.

Start by locating the S3StateStore implementation and reviewing how it passes configuration to S3LevelDown; compare its options with the linked Min.io example. Done means the state store accepts a custom endpoint and can be configured for S3-compatible services such as DigitalOcean Spaces, with the relevant tests passing.

Written by the indexing model from the issue text.