Expose image transition to consumers of flow()

**Glide Version**: 4.15.1
**Integration libraries**: ktx-1.0.0-alpha.3

**Use case background**:
Can `transition` be added to `Resource`? I'm consuming images using `RequestBuilder#flow()` and would like to inspect the `transition` for my usecase.

**Glide load line / `GlideModule` (if any) / list Adapter code (if any)**:
```kotlin
Glide.with(context)
.load("https://example.com/image.jpg")
.flow(Target.SIZE_ORIGINAL)
.collect {

}
```