Add condition to check for new image instead of comparing by imageUrl

## 🚀 Feature Requests

I notice in CachedNetworkImageState, you're using the imageUrl for checking if image has been changed
```
void didUpdateWidget(CachedNetworkImage oldWidget) {
if (oldWidget.imageUrl != widget.imageUrl) {
_streamBuilderKey = UniqueKey();
if (!widget.useOldImageOnUrlChange) {
_disposeImageHolders();
_imageHolders.clear();
}
}
super.didUpdateWidget(oldWidget);
}
```
While it would work in most case, but in my case it doesn't work: after loading new avatar, the backend still return the same image url, but I know its content is changed.

### Contextualize the feature
CachedNetworkImage should has a condition to let user force reload image. I know there is `BaseCacheManager.removeFile` but it's used for remove image from the cache.

### Describe the feature
I think about each CachedNetworkImage will has a ForceReload object (like the CacheManager).
In BLoC side, when need to force reload, we will update the ForceReload.
After that, CachedNetworkImageState.didUpdateWidget and will use ForceReload, realize that it needs to force download image.

### Platforms affected (mark all that apply)
- [x] :iphone: iOS
- [x] :robot: Android

Start in CachedNetworkImageState.didUpdateWidget and trace how imageUrl changes reset the stream and image holders. Review BaseCacheManager.removeFile for the existing cache-removal behavior, then determine how a per-widget force-reload condition should trigger a fresh download when the URL is unchanged. Done means users can force a reload for changed image content on iOS and Android.

Written by the indexing model from the issue text.