Add support for JSON index to chartmuseum

[MeurillonGuillaume]

Recently a PR was merged into Helm (helm/helm#12245) where support was added to store repository indexes in JSON format. This is especially helpful in situations where the repository is very large. The problem that was solved here is that parsing the YAML-index is slow, and it could be sped up if the index was parsed as JSON instead.

I have personally noticed our chartmuseum becoming slower over time as it grew, and looked into chartmuseum compatibility with this fix. Some digging in Helm learned me that one of our issues is in fact parsing the index.yaml returned by chartmuseum.

I checked, and chartmuseum has no support for what was added in the mentionned PR yet. I felt like this was a nice option to add for people with large repositories. I'm opening this issue for suggestions, and I'm also submitting a PR with the changes needed to generate a JSON index.yaml.

[scbizu]

Great , I think it will make big sense for large tendency storage . I will look it soon .