[WIP] nydus-image: support small chunks mergence

[hangvane]

Add the support of merging small file chunks into one batch chunk, for ConversionType::DirectoryToRafs, ConversionType::EStargzToRafs, ConversionType::TargzToRafs, and ConversionType::TarToRafs.

basic usage

Add the --batch-size arg to command to enable chunk mergence for supported conversion types:

nydus-image create --bootstrap ~/bootstrap --blob-dir ~/blobs ~/source --batch-size 0x100000

nydus-image create --type estargz-rafs --bootstrap ~/bootstrap --blob-dir ~/blobs ~/source.tar.gz --batch-size 0x100000

nydus-image create --type targz-rafs --bootstrap ~/bootstrap --blob-dir ~/blobs ~/source.tar.gz --batch-size 0x100000

nydus-image create --type tar-rafs --bootstrap ~/bootstrap --blob-dir ~/blobs ~/source.tar --batch-size 0x100000

This PR is related to #884, #885, dragonflyoss/Dragonfly2#1858