This avoids having to shell out to the `helm2` binary while still
maintaining the same functionality. It aligns with the approach
used in `helm/v3`.

This commit achieves three things:

1. The logic for resolving the full chart URL has been moved to the
   client version implementations, adding two new methods `Pull` and
   `PullWithRepoURL`. The latter implements the logic for retrieving
   the full chart URL that was earlier present in the `chartsync`
   package itself. This ensures the accurate repositories index is
   used when we look for chart repository credentials.
2. The newly introduced `Pull` method makes use of the
   `downloader.ChartDownloader` for that Helm version. This makes it
   easier to integrate e.g. the pull of OCI charts once this feature
   becomes available in Helm v3. It also provides some groundwork for
   working with named repositories which we may want to use later on
   when we have introduced the Custom Resource for manging Helm
   repositories, as it now understands the `repo/name` format.
3. The chartsync now maintains a chart cache per Helm version by
   appending the `helm.Version()` to the `base`.