[helm] Add Helm Plugin

[ricoberger]

Is your feature request related to a problem? Please describe

Helm is a popular package manager for Kubernetes, so it would be nice if we can also add support for Helm. This way we can allow users to manage their Helm releases via the kobs.

Describe the solution you'd like

A new plugin helm should be added, which shows a list of Helm releases. This list can then be filtered by clusters and namespaces. When the user selects a Helm release from the list a details panel is opened, with the current status, values and history of the release. Besides that a user should also be able to rollback the version of a release and maybe it should also be possible to delete a release. It should be possible to define these action on the user and team level via the plugin permissions when authentication is enabled.

For the panel implementation of the plugin it should be possible to show a list of Helm releases (by clusters / namespaces) or a single Helm release, which can be selected via cluster / namespace / name.