`crane append` to all images in an index

crane append -f foo.tar -b ${MULTI_ARCH_IMAGE} --platform=all should append the contents of foo.tar onto each image in the base index.

crane mutate --entrypoint=foo ${MULTI_ARCH_IMAGE} --platform=all should modify the entrypoint for each image in the base index.

Similarly crane mutate --platform=linux/amd64,linux/arm64 should pull out only manifests matching those platforms (failing if they aren't matched), and mutate them.

Perhaps more controversially:

crane mutate --entrypoint=foo ${MULTI_ARCH_IAMGE} --platform=linux/arm64 should modify the entrypoint for only that platform, but update it in-place inside the multi-arch index manifest, instead of pulling only that image, modifying it, and pushing only that image.

This would be a change in behavior which might surprise folks, so maybe hide this behind a flag, maybe --in-place?

@jonjohnsonjr wdyt?

Start with the crane append and crane mutate CLI entry points and trace how platform selection and multi-architecture indexes are handled. Define the expected behavior for all-platform, selected-platform, and in-place updates, including whether a flag is required; done means the commands update the intended manifests without changing others.