Nydus: can we add support for parent bootstrap?

[liubogithub]

This "parent bootstrap" is similar to what "FROM" of docker file offers, it makes it possible to use the existing blobs or chunks when building a new image.

[imeoer]

This is essentially a data deduplication problem, currently, the nydus driver only supports inter-layer deduplication, not global deduplication, we can provide an option to specify the base image, based on the chunks information in the image to implement global data deduplication.