Expose the functional bean registration API via SpringApplication

SpringApplication makes it hard to get hold of the application context before it's refreshed. @michael-simons came up with this:

SpringApplication springApplication = new SpringApplication(Application.class);
springApplication.addInitializers((GenericApplicationContext ctx) -> {
    ctx.registerBean(Greeter.class, Greeter::new);
});
springApplication.run(args);

It would be nice if the initialiser wasn't needed and the API provided an easier way to access the context before it's refreshed.

Start by reading SpringApplication and its addInitializers API, then inspect how the application context is created and refreshed. The issue is complete when the functional bean registration API can be used through SpringApplication before refresh without requiring an initializer, with coverage for the proposed registration flow.