Add onStart callback to SwiftUI.LazyImage

Please add `onStart` to `SwiftUI.LazyImage`.

See an example below.

```swift
  @State private var isLoading: Bool = false

    Button {
      viewStore.send(.tapped)
    } label: {
        LazyImage(url: viewStore.imageURL) { state in
           ...
        }
        .onStart { isLoading = true }
        .onCompletion { _ in isLoading = false }
    }
    .disabled(isLoading)
```