PrivateFile::stream_content doesn't honor the index for inline content

[fabricedesre]

PrivateFile::stream_content() takes an index parameter that is passed to the private forest content when the file content is external. However in the case of inline content, the whole vector is yieled in https://github.com/wnfs-wg/rs-wnfs/blob/6df4a6bf89a71010b7bc15c132512da979ebc438/wnfs/src/private/file.rs#L489

I think we should instead yield data[index..].to_vec()