Move DDS support into image-extras

[Shnatsel]

Now that https://github.com/image-rs/image-extras exists, our read-only DDS support should be moved into that repo.

[RunDevelopment]

Hi! I'm the author of #2461 and dds.

Should I move everything in #2461 over to image-extras or is the more complete DDS support I implemented in dds more fitting for image itself? I'm asking because the incomplete state of the current DDS support may have been a deciding factor for its transfer to image-extras.

Just let me know how you want to do it.