[Cluster Chart] Allow loading S3 keys from secret

[KevinGimbel]

Hello! It looks to me as if loading S3 credentials from a secret is not supported.

These two config values should allow loading using a secretRef

backups.s3.accessKey
backups.s3.secretKey

I'd like to be able to define them like so:

backups:
  s3:
    credentials:
      secretRef:
        name: my-credential-secret

and then have the ACCESS_KEY_ID and SECRET_ACCESS_KEY inside the secret.