Add ability to complete helm charts from different sources before the helm package is called

[wusik]

When building a helm chart, we need to add additional resource files that lie outside the original helm chart folder. For now, we use WA by copying the files we need into the source helm chart folder and deleting them when the build task is over.

[jekkel]

It shouldn't be very hard since we're already using a copyspec internally for an initial copy to render expansions. I can think of adding a method allowing to enqueue a closure (or action) to be applied on this copyspec befor it gets used. Would that do the trick for you?