Add a new blob-ref type into Git data model

Motivation

After &1487 (closed), we should have Git being able to partial clone from standard git-remote and then selectively download large blobs from a promisor-remote separately.

In Yandex presentation on Arc, their internal VCS for their monorepo, it's said that a BlobRef data type could be added to git which represent a file inside a tree object but with content being split into smaller blobs for faster concurrent download.

This should help enable downloads of large blobs from promisor-remote to be parallelized.

Reference

https://habr.com/ru/company/yandex/blog/482926/ (Google translate works well here)