Validate at build time that `UnleashSubscriber` and `ToggleBootstrapProvider` are `@ApplicationScoped` or `@Singleton`

[gwenneg]

Since the Unleash client is an @ApplicationScoped bean and the extension is passing to the client builder the CDI beans that implement UnleashSubscriber or ToggleBootstrapProvider, the processor should validate at build time that these beans are @ApplicationScoped or @Singleton.