S3 prefix for tidb-backup

[tirsen]

Feature Request

Is your feature request related to a problem? Please describe:
We'd like to have a single S3 bucket for all of our backups with a prefix/sub-path for each cluster.

Describe the feature you'd like:
The tidb-backup helm chart currently does not support specifying a prefix. We'd like to be able to specify a prefix in the config:

backup to or restore from s3 bucket, the backup path is in the form of -

s3:
  bucket: "tidb-backups"
  prefix: "my-tidb-cluster"