Revisit UX for singleton

[yim-lee]

Right now singletons are registered during system setup:

let system = ActorSystem("test") { settings in
    settings += ActorSingleton("greeter", greeterBehavior)
}

But the shortcomings of this design becomes apparent rather quickly.

1. Users might end up copy/pasting the same logic in tests, which is annoying
2. It leads to disconnection between singleton and non-singleton actors

We should consider the other approach where settings contains just the flag to enable the plugin and allow users to spawn singletons anywhere.

[ktoso]

Yeah it seemed like a good idea and simpler at the time... I'm likely guilty for wanting to keep it in this simplest form, but seems its too limiting indeed.

So what we can end up with is perhaps this:

import ActorSingleton 

let system = ActorSystem() { settings in 
    settings.plugins += .actorSingleton // extension Plugin { let actorSingleton I guess
}

and usage like

import ActorSingleton 

let ref = system.singletonRef(of: String.self, "uniqueIdentifier") // just proxy
let ref = system.singletonRef(of: String.self, "uniqueIdentifier", behavior: behavior) // just proxy or can host it

let ref = system.singletonActor() // same for actorables

Something like that?