Draft: support dependency values from Helm Chart archives

[justenstall]

Working on support for Helm Chart archives (<CHART>-<VERSION>.tgz files) for dependency values.

Use case: attempting to use helm-docs in CI pipelines, and the only way I can get it to generate docs for dependency values without vendoring the dependency Charts is by removing the "repository" fields from dependencies. I would like to keep the repository information in Chart.yaml so it is included in the generated README.

With this feature, I can run helm dependency build on the target Chart and helm-docs --document-dependency-values will be able to document the values.

This could be a stop-gap, it might be better to just find a way to use Helm's own Chart loaders. If the maintainers like this idea but would prefer to use Helm's loader I can set that up instead.