BitmapDescriptor custom markers

I've tryed to add a custom marker on the map. I can clearly say that some applications using something just like `BitmapDescriptor.markerAnnotation` but there is a custom black icon in the middle of it. Thoose apps written by swift I can say. But if there is a custom marker option, we can do the same too via flutter.

Why not add a constructor for marker that has parameters of hue, icon, size of icon (customizations for icon) etc.

---

Also there is a problem about `BitmapDescriptor.fromAssetImage` and byte version of it. That we can not change the size of the image we are using. After examined the package's code I've tryed this Bitmap to make `scale` work:

```dart
BitmapDescriptor.fromAssetImage(
const ImageConfiguration(
devicePixelRatio: 0.00001,
),
'assets/images/logo/brand/black.png',
mipmaps: false,
).then((bitmap) {
_bitmap = bitmap;

setState(() {});
});
```

But there is no differance at all! that image is 230x230 pixels and its covering the half of the screen no mather what I do. I clearly need a scaling function or differant sizes of this asset for every single device pixel ratio...