Allow chained BeanRegistry registration

[philwebb]

Whilst looking at the new BeanRegistry API it struck me that it might be useful to allow several register calls to be chained together. For example, I might have some custom utility that is itself implements BeanRegistrar. If we add a BeanRegistry.register(BeanRegistrar registar) method I think it will help user to compose registartions.

E.g.:

class MyBeanRegistrar implements BeanRegistrar {

    void register(BeanRegistry registry, Environment env) {
        registry.register(MyCustomScannerThatIsAlsoABeanRegistrar.scan("some.package.for.things.i.know"));
    }

}

[sdeleuze]

Thanks Phil, merged, looks like a useful additional capability.