new feature: expose metadata access for raw list entries

### Feature Description Expose a supported raw-layer API for accessing and modifying metadata on `oio::Entry`, such as `metadata()`, `metadata_mut()`, or `into_parts()`. ### Problem and Solution External layers can forward entries returned by `List::next`, but cannot inspect or modify their metadata because the current metadata accessors are `pub(crate)`. This limits layers that need to adjust listed entry metadata, such as filling missing fields, rewriting content length, filtering user metadata, or preserving backend-provided metadata while changing selected values. Current workarounds are to drop metadata, call `stat` per entry, or route through public `Operator::list` / `Entry` APIs instead of working directly with raw list entries. Please expose metadata access for raw list entries so external layers can transform list metadata without losing backend metadata or issuing extra `stat` calls. ### Additional Context _No response_ ### Are you willing to contribute to the development of this feature? - [x] Yes, I am willing to contribute to the development of this feature.