Allow selecting existing media items when using Uploadcare

**Is your feature request related to a problem? Please describe.**

Currently, adding images and files via uploadcare works great, however if you need to add the same image/file in multiple locations, or change the image - a new file gets uploaded to uploadcare with the existing file being left.

**Describe the solution you'd like**

I think it would be great if when using uploadcare, on selecting to add an image or file, one of the options on the left hand side showed items already in your media library (like the normal media library integration does).

This way, you can manage the files stored on uploadcare, select existing files to be used in multiple places, or delete a previous file before adding a replacement.

**Describe alternatives you've considered**

This functionality is present in the native media library, however I’ve needed to use uploadcare due to its ability to resize images via an api (so clients don’t need to know how to resize images).