Add thumbnailImage projection

Is your feature request related to a problem? Please describe.
As it stands, on iOS the plugin retrieves the full size image, which can slow things down considerably.

Describe the solution you'd like
When displaying contact images as thumbnails, it would be much more efficient to retrieve CNContactThumbnailImageDataKey instead of CNContactImageDataKey.

I'm happy to give it a shot on iOS. I'd simply add a new projection for thumbnailImage and implement the Swift plugin accordingly. I have no experience with Android dev unfortunately.

Describe alternatives you've considered
We could:

• resize the images and store them in a cache. Wildly impractical and overkill.
• write a quick and dirty plugin which only performs thumbnail image retrieval. An hour of work tops, but community wouldn't benefit and our codebase would be that much dirtier.

Start by locating the existing image projections and the Swift iOS plugin implementation mentioned in the issue. Add the thumbnailImage projection using CNContactThumbnailImageDataKey, then verify that iOS contact thumbnail requests use the thumbnail data rather than the full-size image.