[Spec] Use explicit registration

[StephaneDelcroix]

Description

Use explicit registration, or avoid registration (like for slim renderers) instead of scanning all assemblies for assembly attributes for renderers, image handlers, stylepropery, dependency serivces, fonts, ...

As those need to be registered with extra information, the most portable way is to provide a service that you'd acquire on the IServiceCollection, then add the handlers to the service

No API provided at this time, as this covers multiple scenarios

Difficulty :low

[rmarinho]

the current proposed api is IAppHostBuilder will provider ways to register delegates that allow to register handlers , fonts or other things

public interface IAppHostBuilder : IHostBuilder
{
	IAppHostBuilder ConfigureHandlers(Action<HostBuilderContext, IServiceCollection> configureDelegate);
	IAppHostBuilder ConfigureFonts(Action<HostBuilderContext, IFontCollection> configureDelegate);
	IAppHostBuilder ConfigureImageSources(Action<HostBuilderContext, IImageCollection> configureDelegate);
}

To get the services the handler with expose a IServiceProvider

 handler.Services.GetRequiredService<IFontManager>();