add an annotation with layer contents

[tych0]

Along side the StackerContentsAnnotation, it would be nice to have a LayerContentsAnnotation = "com.cisco.layer_contents", which is a list (maybe in json format) of the layer contents. This is especially easy for stacker to generate since in the btrfs backend it has the mtree information, and in the overlay backend it has the overlay/ directory with exactly the contents.

One question here is what to do about whiteouts. I see two possible options:

1. have the json object be a map, so {"contents": ["/usr/bin/..."], "whiteouts": ["/etc/config.conf"]}, with a list of the whiteouts
2. generate filenames as in the oci spec, so .wh.foo

Either option will require some processing of the contents of the mtree/overlay, so it's probably down to whichever is easier for tools/humans to consume.