[FEATURE] Create ztoc in a single pass

[Kern--]

Description

The current implementation of ZTOC building first creates zinfo from the layer file, then creates a toc from the file:

soci-snapshotter/ztoc/ztoc_builder.go

Lines 93 to 101 in 99fc201

	compressionInfo, fs, err := b.zinfoBuilders[opt.algorithm].ZinfoFromFile(filename, span)
	if err != nil {
	return nil, err
	}
	toc, uncompressedArchiveSize, err := b.tocBuilder.TocFromFile(opt.algorithm, filename)
	if err != nil {
	return nil, err
	}

The implentation of Zinfo building first creates the Zinfo from the layer file, then gets span digests from the file.

soci-snapshotter/ztoc/zinfo_builder.go

Lines 40 to 54 in 99fc201

	index, err := compression.NewZinfoFromFile(compression.Gzip, filename, spanSize)
	if err != nil {
	return
	}
	defer index.Close()
	fs, err = getFileSize(filename)
	if err != nil {
	return
	}
	digests, err := getPerSpanDigests(filename, int64(fs), index)
	if err != nil {
	return
	}

The full path for compressed layers is:

1. Read and decompress the layer file to create zinfo
2. Read the compressed layer file to create span digests
3. Read and decompress the layer file to create the TOC

We read the file 3 times and decompress it 2 times.

Describe the solution you'd like

We should be able to do this in a streaming context:

1. Read the file, decompress it to create the zinfo and span digests at the same time
2. Use the decompressed stream to create the TOC

That way we should be able to read the file once, decompress once, and read the uncompressed stream once.

Describe any alternative solutions/features you've considered

No response

Any additional context or information about the feature request

Related to:
#366
#76