Allow S3 Storage Provider to support sts_endpoint

**Is your feature request related to a problem? Please describe.**

Minio has added [sts support](https://min.io/docs/minio/kubernetes/upstream/developers/sts-for-operator.html) to work with the k8s service account JWTs. Currently because it is a different url to hit for sts, we are unable to leverage this feature.

**Describe the solution you'd like**

objstore has added the sts_endpoint in their [configuration file](https://github.com/thanos-io/objstore/blob/main/providers/s3/s3.go#L143), if that could be added to the s3 configuration that would be great!

**Describe alternatives you've considered**

manually creating a service account.

**Additional context**

https://min.io/docs/minio/kubernetes/upstream/developers/sts-for-operator.html
https://github.com/thanos-io/objstore/blob/main/providers/s3/s3.go#L143