OMERO.matlab: add function to retrieve thumbnails (see #10384)

Can be tested with

```
getThumbnail(session, image)
getThumbnail(session, image, x, y)
getThumbnailByLargestSide(session, image)
getThumbnailByLargestSide(session, image, size)
getThumbnailSet(session, imageIDs)
getThumbnailSet(session, imageIDs, x, y)
getThumbnailByLargestSideSet(session, imageIDs)
getThumbnailByLargestSideSet(session, imageIDs, size)
```