Image decode complete notification

## 🚀 Feature Requests

Due to some needs of the project, we need to monitor the loading time of pictures, not only downloading, including decoding. That is, we need to know how many times to display a set of pictures, from the begin loading to render complete (rendering monitoring may be difficult, then use the decoding completion time is ok) .

I think the property _isLoaded of ImageHandler is very useful, how to expose it. Actually, we want to use(or get) it in CachedNetworkImage

### Contextualize the feature

### Describe the feature

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

Start by locating ImageHandler and CachedNetworkImage, then trace how image loading and decoding currently become observable. Clarify the notification or exposed state expected by consumers and define completion as decoding finished; the feature is done when callers can reliably observe that point.

Written by the indexing model from the issue text.