feat: add git chart downloader
This is meant to be compatible with the helm-git plugin.
TL;DR: The plugin has been reimplemented here since it relies on helm commands khelm does not support and since it doesn't build git charts as effective as khelm. khelm uses go-git to keep the image small and self-contained: the git binary would increase the container image size by 15m while using go-git increases the size by 3m only. However currently go-git does not support sparse checkouts.
I needed this for a previous approach but I don't need it anymore. I am not sure whether it should still be merged since it adds more features to the binary that may be rarely needed but that increase the binary size.
Please vote with thumbs up/down whether or not you want this PR to be merged.