[msbuild] Add a public 'CompileImageAssetsDependsOn' property. Fixes x…

…amarin#16065.

Add a public 'CompileImageAssetsDependsOn' property, so that MAUI can inject tasks that
must be completed before we compile image assets:

```xml
<PropertyGroup>
	<CompileImageAssetsDependsOn>
		$(CompileImageAssetsDependsOn);
		ResizetizeCollectItems;
	</CompileImageAssetsDependsOn>
</PropertyGroup>
```

Fixes xamarin#16065.

msbuild/Xamarin.Shared/Xamarin.Shared.targets

@@ -792,6 +792,7 @@ Copyright (C) 2018 Microsoft. All rights reserved.
 
 	<PropertyGroup>
 		<_CompileImageAssetsDependsOn>
+			$(CompileImageAssetsDependsOn);
 			$(_CompileImageAssetsDependsOn);
 			_DetectAppManifest;
 			_DetectSdkLocations;