Support copying multiple platforms.

[ameir]

crane index filter is a great way to copy multi-platform images without copying undesired platforms. Unfortunately, that command requires that the source image support multiple platforms (i.e., has a manifest list/index). In a scenario where you may be programmatically copying images, some source images may not host a manifest list/index, and additional checks must be performed to decide between crane index filter and crane copy.

This PR updatescrane copy so that it behaves similar to crane index filter, but does not require a manifest list/index. In a single-platform scenario, it behaves much as it did before. In a multi-platform scenario, it filters the manifest list/index to the desired platforms, and copies only those layers.

Example command:

crane copy --platform linux/amd64 --platform linux/arm64 redis localhost:58273/redis

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[ameir]

@imjasonh / @jonjohnsonjr do either of you have cycles to take a look at this PR? Thanks!