[java-storage-nio] Support for gcsfuse symlinks.

When using `gcsfuse`, we could have symlinks in mounted buckets. When looking at these symlinks, they are stored in GCS as empty files with metadata pointing to the target file symlink. Currently, if I access to this file with the `java-storage-nio interface`, it is seen as an empty file.

It would be great to support them and when we do a NIO operation to a symlink it does to the target file or return a FileNotFoundException when the target path doesn't exist.