feat: add option authType for s3 storage

[everpcpc]

And add support for IAM and ENV auth for AWS S3.

Would possibly fix #386

[turt2live]

fwiw, the way s3 is handled is changing to be more cost effective. Auth will likely be part of that, but just a heads up that this PR might take a while to land because I want to make sure the rest of the stuff works before introducing more changes to worry about.

[everpcpc]

@turt2live Hi, i've reverted the Dockerfile change and the minio-go upgrade.

[turt2live]

Thanks for working on this. The codebase has changed pretty dramatically over the last year, but this is still something I'm interested in landing.

A current concern with this approach is users with multiple datastores can't specify multiple types of auth to use. If two datastores use environment variables, for example, only one will work (or both have to share those credentials, which is uncommon). Would it be possible to support multiple datastores in this setup?

[everpcpc]

Official AWS client has profile support for these cases, but it seems minio-go dose not support that.