Add support for 'condition' matching on @Subscriber similar to Spring's @EventListener

[longwa]

Spring's @EventListener has a condition attribute that can use an SpEL expression to match:

@EventListener(condition = "#creationEvent.awesome")
public void handleOrderCreatedEvent(CreationEvent<Order> creationEvent) {
}

It would be super handy if @Subscriber could add the same support. This would allow for a better replacement to the pattern matching support that @Selector used to provide with the legacy reactor bus.

Also, we have an enumeration for our eventTypes to make it easier to find and maintain. Having the SpEL expression would allow us to match based on the actual enum value instead of needing to duplicate the string for the eventId (since you can't say value = MyEnum.name() in an annotation).