Support SOCI index as alternative to estargz

[sluongng]

https://github.com/awslabs/soci-snapshotter/blob/main/docs/index.md

The ztoc and soci index could be created during build time and does not require rebuilding existing images to a new format.

[malt3]

Hey, thank you for creating this feature request.
I think this would be a great addition. We welcome external contributions and added this to our backlog.
Note to other users: Please upvote features you would like to see implemented so we can work on important features first.

[malt3]

Some notes after looking at the spec in more detail:

• I think we want to implement v2 of the spec, which is meant for build-time conversion and produces a single artifact with the container image and lazy loading metadata. Anything I'm missing? (I think that's what you are referring to in the original feature request, just double checking)
• we should approach this bottom up and start by generating ztoc as layer metadata. The existing compressors are in pkg/compress (compressor.go and estargz.go). A ztoc-enabled compressor needs to implement either api.Appender or api.TarAppender.

[malt3]

The SOCI snapshotter depends on zlib and requires cgo.
That will make it much harder to ship prebuilt binaries ☹️.