Support AWS v4 authentication for S3-compatible backend

[aldy505]

https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-authenticating-requests.html

Should support this since most S3-compatible backend that self-hosted users uses requires a certain authentication using at least "S3 access token" + "S3 secret token". We can defaults the region name to auto.

[jan-auer]

We'll keep this open as a feature request. Right now we're not actively looking to expand S3 support and might back it out.

[aldy505]

Don't back it out! I'll submit some PRs when the time comes.