Add annotate command for adding annotations for indexes and images

[badstreff]

I would like to be able to add annotations to manifests (index or image) in a registry. Originally I was going to extend the mutate command to do this somehow but it makes a lot of assumptions that the manifest being mutated is an image and they code was getting pretty muddy adding logic to handle either.

[google-cla]

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[badstreff]

@brandonmichigangithub Mind giving this a review? My first contrib to the project here so please let me know if there is a better way to bump PRs. (I looked for a community slack but couldn't find one)