FileIO parameter should be arrow::fs::FileSystem ?

The ManifestWriter/ManifestListWriter API uses the` std::shared_ptr` parameter for the IO interface,
which actually should be an internal `arrow::ArrowFileSystemFileIO` (not publicly accessible).
It appears that only the `fs()` is actually utilized.
```c++
auto io = internal::checked_pointer_cast(options.io);
ICEBERG_ARROW_ASSIGN_OR_RETURN(auto output, io->fs()->OpenOutputStream(options.path));
```