laleksiunas has created PR #874: Add partial file retrieval when using GridFS stream download in mongo-rust-driver

Issue Text:
Implemented support for partial file retrieval according to GridFS [spec](https://github.com/mongodb/specifications/blob/master/source/gridfs/gridfs-spec.rst#partial-file-retrieval) for `GridFsBucket::open_download_stream` and `GridFsBucket::open_download_stream_by_name`.
*Unfortunately, there is a breaking change in `GridFsBucket::open_download_stream` interface - additional options argument to accept start/end range.*

• [ ] start and end values are non negative (ensured by design using u64)
• [ ] validate whether start is less than or equal to end
• [ ] validate whether start and end are less than or equal to file length
• [ ] file chunk size is being used to skip and limit chunks when performing partial reads