Support download mode file in helm chart

[DrPsychick]

from #1724

[DrPsychick]

If we add this, I think the chart should support all DownloadMode options.
https://github.com/gomods/athens/blob/43ee4259e31561f7167d7aadbd80d9a2d7884fd8/config.dev.toml#L248

The file: does not need to be explicitly supported in the Chart as a ConfigMap, as one could then use custom:<b64-encoded-hcl> or provide a existingConfigMap which contains the file.

[dsloanather]

Any progress on this?

[DrPsychick]

This is one of the things I would tackle next, first I want to focus efforts on getting Athens up to date again (release v0.12.0).

I'm happy to review if you're willing to contribute a PR.