[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.

Start at the java-storage-nio interface and trace how GCS objects with gcsfuse symlink metadata are handled during NIO operations. Compare the behavior for an existing target and a missing target; done means symlinks resolve to their target or produce FileNotFoundException when the target path does not exist.

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