Holds a list of blake3 digest for individual chunks (and their sizes).
Is able to construct a Reader that seeked to a certain offset, which
is useful to construct a BlobReader (that implements AsyncSeek).
ChunkedReader provides a chunk-aware BlobReader, so allows reading and
seeking into a blob.
It internally holds a ChunkedBlob, which is storing chunk information
able to emit a reader seeked to a specific position whenever we need to seek.