Gather charts releases from external git repos releases to single Helm repository

[MurzNN]

Current action parses only locally available charts. But it's very inconvenient to manage large single monorepo with plenty of different charts, because we've got a mess of PRs, commits and issues from different charts in one plain place.

To decrease such mess Helm monorepo owners must enforce rules to force prefixes with chart name to all commit messages, PRs and issues, tackle with changelog generators to make separate changelogs per each chart.

So managing each chart (or logical group of related charts) in separate git repository is much more convenient for all!

But with this way we've got problem with generating a Helm repo using chart-releaser-action, that must somehow aggregate all charts from external git repositories in single repository.

So will be good to implement ability to gather chart releases from listed external Helm Chart git repositories when generating packages index file.

[MurzNN]

Examples of problems with large Helm Chart monorepos you can lookup here:

• https://github.com/bitnami/charts/issues?q=
• https://github.com/bitnami/charts/commits/master
• https://github.com/k8s-at-home/charts/issues?q=
• https://github.com/k8s-at-home/charts/commits/master

List of issues, PRs and commits are plain mess with mix from different charts, without ability to filter them by individual chart!