[Feature request]: Allow using BitmapDescriptor as icons to markers

### Is there an existing issue for this?

- [x] #416

### Use case

I have implemented Google Navigation SDK in my flutter app, earlier I was using

BitmapDescriptor icon = await getRoundNetworkImage(
url: imageURL,
size: MyScreenScaler.myAdaptiveDimension(50),
);

and adding it like

Marker(
markerId: MarkerId("${DateTime.now().millisecondsSinceEpoch}"),
position: LatLng(latlng.latitude, latlng.longitude),
icon: icon,
onTap: null,
),

which would show the profile photo of the user on the map, making an immersive experience for the user, but with google_navigation_flutter: ^0.6.2 I am unable to set BitmapDescriptor and have to use its ImageDescriptor through which I am unable to load the imageURL icon on the map, maybe I am missing something or support of BitmapDescriptor will solve this challenge

### Proposal

Allow using BitmapDescriptor and not just ImageDescriptor to set as icons to markers