Chartmuseum storage support for UnionFS

First of all, chartmuseum is a great tool for self hosting Helm charts. Thank you for all the hard work!

Our CI pipeline pushes a new version of charts every time there's some change in the git repo containing the charts. As the commits and new PRs pile up, it creates a lot of redundant charts which need to be deleted after some time (either manually or through some automation).

I was thinking it'd be great to have something akin to layers in Docker so that only the diff of the new chart version is stored every time I push an updated version of the chart instead of storing a new copy.

I am not sure how easy/difficult this'd be to implement. Also, storage size won't be a big problem unless you have a lot of charts, since the storage size of a chart on an average is not that big.