exiv2extractor: add support for Canon CR3 raw image

The image/x-canon-cr3 mime type is registered via recently released shared-mime-info 2.2

I am unsure whether to add a test-file or not, because .CR3 images are very large (I have an example which has 14MB) and we do not have samples for all formats anyway.

I tested on my computer and the metadata are shown in dolphin for .cr3 files now.