[Fabric] Add image caching

[marlenecota]

Using this to track image caching on Fabric since it isn't covered by API parity

[chrisglein]

Question: How often are we caching on Paper? If you give RNW a uri, and then RNW itself loads it into a memory stream, and only after that hands it off to XAML... that bypasses XAML's image cache.

Of course for Fabric, we never benefit from XAML's image cache. So cases like offline images will need some attention.