[google_maps_flutter_platform_interface] Add improvements to BitmapDescriptor API (#6687)

This PR adds improvements to BitmapDescriptor API for google_maps_flutter_platform_interface 

* Adds better support for marker size and scaling behaviour with `AssetMapBitmap` and `BytesMapBitmap`.
* Deprecates `BitmapDescriptor.fromAssetImage` in favor of `BitmapDescriptor.asset` and `AssetMapBitmap.create`.
* Deprecates `BitmapDescriptor.fromBytes` in favor of `BitmapDescriptor.bytes` and `BytesMapBitmap`

This is prequel PR for: #4055
Containing only changes to `google_maps_flutter_platform_interface` package.

Follow up PR:s will hold the platform and app-facing plugin implementations.

Linked issue: flutter/flutter#34657

Author: jokerttu

packages/google_maps_flutter/google_maps_flutter_platform_interface/CHANGELOG.md

@@ -1,5 +1,8 @@
-## NEXT
+## 2.7.0
 
+* Adds better support for marker size and scaling behaviour with `AssetMapBitmap` and `BytesMapBitmap`.
+* Deprecates `BitmapDescriptor.fromAssetImage` in favor of `BitmapDescriptor.asset` and `AssetMapBitmap.create`.
+* Deprecates `BitmapDescriptor.fromBytes` in favor of `BitmapDescriptor.bytes` and `BytesMapBitmap`
 * Updates minimum supported SDK version to Flutter 3.16/Dart 3.2.
 
 ## 2.6.0