Find a way to show configuration files diff between different versions of an image

When storing configuration files in Git, it's easy to show diff between different versions. When storing configuration files in a docker image, we will need an imgpkg sub command to show the diff (which is similar to the --diff-changes of kapp)

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Start by reviewing the imgpkg subcommand structure and the behavior referenced by kapp's --diff-changes option. Define how configuration files from two image versions should be compared and what output and tests would establish that the feature is complete.

Written by the indexing model from the issue text.