Finer grain control over ActorSystem startup

[Arkatufus]

Is your feature request related to a problem? Please describe.

Right now, there is no proper programmatic way to defer/modify ActorSystem hosted service startup, everything is started immediately on host application start up.

Describe the solution you'd like

A way for users to modify/override the IHostedService.StartAsync() and IHostedService.StopAsync() methods.

[Arkatufus]

Possible solution:

• Convert AkkaHostingService to public abstract and expose some of the properties as protected.
• Add a new AddAkka extension method that have this method fingerprint:

        public static IServiceCollection AddAkka<T>(
            this IServiceCollection services,
            string actorSystemName,
            Action<AkkaConfigurationBuilder, IServiceProvider> builder)
            where T: AkkaHostedService