feat(io): add native in-memory FS

[alessandro-nori]

Summary

• Adds MemFS: a pure Go in-memory IO implementation with no external dependencies
• Replaces gocloud.dev/blob/memblob usage in inferFileIOFromSchema with MemFS
• MemFS implements WriteFileIO and ListableIO; buckets are identified by the mem://bucket/ URI hostname and persist for the process lifetime

Part of #696.