Add custom files into the image

[drew-viles]

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

A user has requested that we support adding custom files into the image builder code. the EKS Anywhere supports this already so we should be able to replicate something similar to what they have done over there in this project.

https://anywhere.eks.amazonaws.com/docs/osmgmt/artifacts/#mounting-additional-files

Describe the solution you'd like

A way of adding additional files from a src to a dst. The user, group etc should also be configurable.

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/kind feature