Add support for standard AWS authentication in s3 backend.

Currently we require users to provide AWS credentials in plain text. However, AWS supports many other authentication methods.

All of AWS tools and sdks support all of those auth methods natively and so AWS users are very familiar with them and often expect them to work in other products.

We use Thanos S3 configuration bits an pieces. Thanos also supports a mode where authentication settings are picked up from user's environment (e.g ~/.aws/credentials, instances metadata, session profiles, etc). The parameters is called [`aws_sdk_auth`](https://github.com/thanos-io/objstore#:~:text=However%20if%20you%20set%20aws_sdk_auth%3A%20true) We could probably adopt whatever Thanos is doing pretty easily.

There's [Slack thread](https://pyroscope.slack.com/archives/C01FJRYENPQ/p1693347838548019) where a community member is asking about this functionality.