Option to specify what files should be copied over from the parent chart into one or more subcharts

**Is your feature request related to a problem? Please describe.**
n/a

**Describe the solution you'd like**
Provide an optional field that lets the user specify what artifacts (files/dirs/templates) should be copied over from the parent chart into one or more staged sub-charts. The input can be a list of file paths or patterns.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
In some cases the subcharts may need to access the `*.tpl` files from the parent chart, in which case we must make the files available in the subchart;'s templates dir.

No files, tests, or entry points are named in the issue. Start by locating the chart-staging logic that creates subcharts, then determine how an optional list of file paths or patterns would be represented and applied. Done means selected files, directories, or templates from the parent chart can be copied into one or more staged subcharts without affecting users who omit the option.

Written by the indexing model from the issue text.