Support other than gzip in tidb-lightning

Feature Request

Is your feature request related to a problem? Please describe:

The Init Container of tidb-lightning is pingcap/tidb-cloud-backup and it seems to have tar xvzf hardcoded into its Command. That means that it'll fail if a different compression scheme is used.

I guess this means the pingcap/tidb-cloud-backup image needs to be changed, but I have no idea where that image comes from.

Init Containers:
  data-retriever:
    Container ID:  docker://42b42f516a3bf5cf66c4c414936a5dd8c7f1266639c7cd0a19fa398782fc75b5
    Image:         pingcap/tidb-cloud-backup:20200229
    Image ID:      docker-pullable://pingcap/tidb-cloud-backup@sha256:d4c61b3573882c3d00e5fee3f911123ed84bdc7235ba8acbbab56560f2433cf1
    Port:          <none>
    Host Port:     <none>
    Command:
      /bin/sh
      -c
      set -euo pipefail
      filename=$(basename s3://kolbe-db-backup-testing/export-20200625-221301.tgz)
      if find /data -name metadata | egrep '.*'; then
          echo "data already exist"
          exit 0
      else
          rclone --config /etc/rclone/rclone.conf copy -P s3://kolbe-db-backup-testing/export-20200625-221301.tgz /data
          cd /data && tar xzvf ${filename}
      fi

Describe the feature you'd like:

Maybe just remove the "z" from the tar command so it auto-detects the compression algorithm?

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

Start by tracing the tidb-cloud-backup init container and the /bin/sh command shown in the issue, including where the pingcap/tidb-cloud-backup image is built. Check how the archive command handles compression, then verify that the init container successfully retrieves and extracts non-gzip archives without breaking gzip support.