[regression/8.0.0-preview.5.8529] Provide API to set ImageSource to the ImageSourcePartLoader

[APopatanasov]

Description

In net7 we are using the ImageSourcePartLoader class to load native images for several properties of type ImageSource. However, after this change this is no longer possible. Now in net8 the source is tightly coupled with the virtual MAUI view.

Because of that you are no longer able to have two (or more) properties of type ImageSource in the MAUI virtual view. We are looking for a way to load properties of type ImageSource without being dependent on the virtual MAUI control.

Public API Changes

You can provide a way to set the ImageSource of the ImageSourcePartLoaded through the constructor for example or with a property.

If this is not an option can you at least make the following extension method public so we can try loading the native images without the usage of ImageSourcePartLoaded:
https://github.com/dotnet/maui/blob/release/8.0.1xx-preview6/src/Core/src/Platform/iOS/ImageSourcePartExtensions.cs
https://github.com/dotnet/maui/blob/release/8.0.1xx-preview6/src/Core/src/Platform/Android/ImageSourcePartExtensions.cs
https://github.com/dotnet/maui/blob/release/8.0.1xx-preview6/src/Core/src/Platform/Windows/ImageSourcePartExtensions.cs

The method is already public for Tizen, so it will be nice to have it for Android, iOS and WinUI as well.

Intended Use-Case

In my custom control I have more than one property of type ImageSource. By implementing the IImageSourcePart interface I can return only a single property that will be loaded by the ImageSourcePartLoaded class.

[ghost]

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