refactor(ofs): Split fuse3 impl into fuse3_opendal

[Xuanwo]

Our users will be able to use:

pub async fn mount(
    self,
    mount_point: impl AsRef<Path>,
    op: Operator,
) -> io::Result<MountHandle> {
    let adapter = fuse3_opendal::Filesystem::new(
        op,
        self.uid.unwrap_or_else(|| nix::unistd::getuid().into()),
        self.gid.unwrap_or_else(|| nix::unistd::getgid().into()),
    );
    Session::new(self.mount_options)
        .mount(adapter, mount_point)
        .await
}

Context: #4451