V2 serialization format for wide columns with blob references

Introduce a new V2 serialization format for wide column entities that
supports storing individual column values in blob files. The V2 format
adds a column type section that marks each column as either inline or
blob-index, enabling per-column blob storage for large values.

Key additions:
- SerializeWithBlobIndices(): serialize entities with blob column references
- DeserializeColumns(): deserialize V2 entities extracting blob column metadata
- HasBlobColumns(): lightweight check for V2 blob references
- GetVersion(): return the format version of a serialized entity
- ResolveEntityBlobColumns(): resolve all blob references in an entity
- Updated Deserialize() and GetValueOfDefaultColumn() for V2 compatibility

Author: xingbowang