FR: Support custom S3-server

[chymian]

Since skypilot supports onPrem clusters, it would also be logical & necessary to support custom/onprem S3-server.

1. there can be a significant speedup instead of pulling the data from S3-cloudservices (AWS/GC/etc), every time a task ist started inhouse, if the S3-bucket is hosted/mirrored inhouse as well.
2. thinking EU Data-Security Laws and company policies: Data has to be keept inhouse!
3. So, it's a missing piece and without it, K8s/onPrem support is not complete and not truly onprem, when one has to store the sensitive company data on a CloudService - just do download it again into my DC.

[romilbhardwaj]

+1. Meanwhile, noting a workaround using local Kubernetes volumes to address perf and privacy considerations - in your ~/.sky/config.yaml, you can specify custom volumes like so:

kubernetes:
  pod_config:
    spec:
      volumes:
        - name: example-volume
          hostPath:
            path: /tmp
            type: Directory

[Michaelvll]

Related to #2687