Invoke Build Callbacks at Lifetime Scope Creation

[alistairjevans]

Following the discussion in #985, these changes allow build callbacks to execute at the point of lifetime scope creation if new ones have been registered.

So, this works:

var scope = container.BeginLifetimeScope(cfg =>
{
    cfg.RegisterBuildCallback(scope => { /* do something */ });
});

The callback will be invoked just prior to BeginLifetimeScope exiting, after any startable components are instantiated (same behaviour as the Container).

As discussed, we now register a component BuildCallbackService that contains the set of callbacks to register; nested scopes then add replacement registrations of that service.

BuildCallbackManager is responsible for executing the callbacks.

Fixes #985

[tillig]

I think this is a nice, elegant way to solve the whole thing and unify the way callbacks are done. Great job!

[tillig]

I think if we can remove the nullable notes we're good.

[tillig]

🎉