new feature: direct IO support in fs

### Feature Description

Support direct io mode in fs backend

### Problem and Solution

By using direct io, we can directly write/read into file system without through page-cache, which is useful in DBMS to control the memory.

### Additional Context

_No response_

### Are you willing to contribute to the development of this feature?

- [ ] Yes, I am willing to contribute to the development of this feature.

The issue targets direct I/O support in the fs backend. Start by locating the fs backend and determine how direct I/O should be exposed and supported across platforms; the issue does not identify files, tests, or a completion criterion, so the API and validation plan need clarification first.

Written by the indexing model from the issue text.