How to precache an image

## Feature request for Cloudinary Flutter SDK
Precache a cloudinary image
## Explain your use case
I wanted to be able to precache an image so that when the CldImageWidget is built, it'll already have the image loaded as it has been precached

## Describe the problem you’re trying to solve
Have an image precached so that it'll be loaded before the CldImageWidget is built, leading to a more responsive feeling

## Do you have a proposed solution?
I was thinking of using something like this (note precacheImage is flutter's builtin method, yet it requires an ImageProvider as it's argument):
`precacheImage(NetworkImage(publicId), context);
`
Instead of NetworkImage, use a cloudinary image provider.