Add support for different types of diffs for the generated-changes/patches directory

[aiyengar2]

Add a field to the package.yaml to allow for different options for patches exported to generated-changes/patches/.

The default (as it is now) would be:

diffOptions:
  type: unified_diff

But we could potentially support other diff types.

Would predominantly just require us to make changes to diff/diff.go.

[aiyengar2]

The purpose for this enhancement is to be able to provide an option to avoid the "diffs on diffs" issue we see when reviewing PRs to a charts repository.