proposal: support small chunks mergence

[imeoer]

Support merging small chunks (including the tailing chunk for big files) into one batch, so that we can compress and decompress the batch to improve the compression ratio, prefetch and io amplify performance.

[imeoer]

From @jiangliu: or consider using the BlobChunkInfoV2Ondisk struct.

[jiangliu]

We have built some infrastructure to support ZRan, which could also be used to support this feature requirement.

https://github.com/dragonflyoss/image-service/blob/master/storage/src/meta/chunk_info_v2.rs#L24

You can reuse or introduce a new struct like this one for ZRan:
https://github.com/dragonflyoss/image-service/blob/master/storage/src/meta/zran.rs#L16

[imeoer]

Related dragonfly issue: dragonflyoss/Dragonfly2#1858