Ability to always download artifacts

### Proposal

The [artifact](https://www.nomadproject.io/docs/job-specification/artifact) stanza in Nomad doesn't "re-download" the files if the allocation has those files already present. For eg: when an allocation is restarted, the files aren't downloaded again. This makes sense for faster startup times of the task and is a nice feature to have. However, if there's another `always_download=true` kind of flag, that will be super helpful for me.

### Use-cases

I've a Go app that can only start if the file is externally downloaded from an external source like HTTP/S3 etc. The ability to fetch the file is left to external orchestration. This file gets updated at every `n` hours, so my CI system has scheduled pipelines to restart the allocation. However this file doesn't get re-downloaded.

### Attempted Solutions

Since we're migrating from K8s, we had initContainers which did that job for us. But here that isn't possible as noted in https://github.com/hashicorp/nomad/issues/9841#issuecomment-940757058. So now the only real option for me is to use `sidecar=true` and after the file is downloaded, for the shell script to not exit, I've to resort to `sleep infinity` which is undesirable :)

What would be the best way out here? I am happy to send a PR for a new flag `always_download` if that is acceptable. Would just need a basic pointer on where to start looking to change this.

Thanks!

Start with the artifact stanza documentation and trace the allocation artifact-download behavior described in the issue. Determine where existing files cause downloads to be skipped and identify the related tests, then define completion as a supported option that fetches artifacts again when an allocation restarts, with coverage for that behavior.

Written by the indexing model from the issue text.