extend the image-selection mechanism to prefer images compressed with the zstd algorithm.

This will help us move forward with zstd and more specifically with zstd:chunked to support partial pull of images.

Link to the design doc: https://docs.google.com/document/d/1IH7ef1oqpAJMh4c7v9n0ya4c_zPU2BnZq02VwoBXAyQ/