Support recording pixel aspect ratio as Density option

[Shnatsel]

The JPEG encoder in image supports indicating the pixel aspect ratio via the density option: https://docs.rs/image/latest/image/codecs/jpeg/enum.PixelDensityUnit.html

This gets encoded as unit value 0 in the final image: https://docs.rs/image/0.25.8/src/image/codecs/jpeg/encoder.rs.html#762-765

The Density in jpeg-encoder supports the None option but does not allow associating values with it.

This is the only blocker for integration into image for which I don't have a PR open: image-rs/image#2636