Support hosts other than AWS for S3 backend

[bo0ts]

Currently only AWS is supported as an S3 host, but some users might want to use their own S3 host (e.g. minio). Either allow setting the --host and associated switches via the secret or allow injection of an .s3cfg file.

[matskiv]

Hello @bo0ts
thank you for this suggestion!
Sorry for not noticing this sooner.

Unfortunately, we are not using any alternative S3 hosts, so I don't think we will be able to implement this in the near future.
But PRs for such a feature are more than welcomed of course.