[OSPP 2024] optimize image by blob compact

[imeoer]

Nydus can optimize the image size by using chunk dict deduplication, but this can result in small blobs, and poorly written Dockerfile can also result in a lot of small blobs, this affects the I/O read amplification, I/O merge, and data prefetch performance during the image runtime.

Possible implementation steps:

• implement nydusify optimize --policy compact-blob --config opt1=val1,opt2=val2 --target $existed-nydus-image --output $new-nydus-image to generate a new image, and compact the blobs referenced by the nydus image.