Allow the easy instantiation of CDI beans

Description

I was wondering if it would make sense to make eager instantiation super easy for users via either an annotation (for per bean control) and / or a config property that would make bean instantation eager by default.

Implementation ideas

We could basically generate the necessary void startup(@Observes StartupEvent event) under the hood - or perhaps tie deeper into Arc to make this controllable.

[mkouba]

Implementation note: the easiest solution from impl POV would be to introduce an interface, e.g. StartupCallback with default no-op startup() method and then in the LifecycleEventRunner iterate over all beans that implement this interface and invoke each of the beans.

[gastaldi]

+1 for a @Startup annotation. In some classes writing an observer method just to initialize (which does nothing) during the startup is annoying