[feature] Output artificat for batch jobs

Hi, we have been working with nomad for some months to test its capabilities for our use case (mostly batch processing). And we found that nomad was missing a feature for artifact management:
it would be really nice to have an output artifact stanza that will upload the batch results to S3 after the job completes. Something like:

```hcl
artifact {
output = true
source = "/output/batch_result.txt"
destination = "s3::https://minio.domain.com/"
options {
aws_access_key_id = "${aws_access_key_id}"
aws_access_key_secret = "${aws_access_key_secret}"
}
}
```

The issue names no files, tests, or entry points. Start by locating Nomad's existing artifact handling and batch-job completion flow, then determine how an output upload to S3 should behave; done should include a defined artifact stanza and successful upload of batch results after completion.

Written by the indexing model from the issue text.