[FR]: js_image_layer should expose 'tar' configurability

[alexeagle]

What is the current behavior?

Users can't exclude things from the generated layer, for example tools that bring in a bunch of fat native binary dependencies.

We made a lot of progress recently with our tar rule in bazel-lib. It seems like js_image_layer needs a refactoring (maybe as part of 2.0)

Describe the feature

Not sure yet, needs design discussion