How can I add image to cache manually.

Hi here, I want to add some images to cache manager manually for CachedNetworkImage. My code below doesn't work so what can I do to achieve that?

```
// storing cache
String imageUrl = 'some url';
String cacheKey = 'some_key';
DefaultCacheManager cacheManager = DefaultCacheManager();
await cacheManager.downloadFile(imageUrl, key: cacheKey);

// render image
CachedNetworkImage(
cacheManager: cacheManager,
cacheKey: cacheKey,
imageUrl: 'imageUrl',
),
```

Start by reading the CachedNetworkImage and DefaultCacheManager usage shown in the issue, especially downloadFile and the cacheKey/imageUrl values. Reproduce the reported behavior in a minimal Flutter app and verify whether the manually cached image is rendered; the issue is done when the supported way to preload and display that image is documented or corrected.

Written by the indexing model from the issue text.