storage: support multiple storage backends

[skyzh]

Currently, RisingLight cannot be compiled or run on Windows, because we are using ReadAt extension of UNIX. As our students might use Windows as their development environment, we need to add new storage backends (for reading).

Basically, all reads are handled in Column structure https://github.com/singularity-data/risinglight/blob/main/src/storage/secondary/column.rs

It's better to change file: Arc<std::fs::File> into an enum, e.g.

pub struct ColumnReadableFile {
  /// For `read_at`
  #[cfg(unix)]
  PositionedRead(Arc<std::fs::File>),
  /// For `file.lock().seek().read()`
  NormalRead(Arc<Mutex<tokio::fs::File>>),
  // In the future, we can even add minio / S3 file backend
}

And we should refactor the whole code path to use ColumnReadableFile instead of Arc<std::fs::File> throughout the storage system.