* INT-4444: Introduce `@Reactive` & `reactive()`
JIRA: https://jira.spring.io/browse/INT-4444
Right now the high-level API creates a `ReactiveStreamsConsumer`
only when the input channel is a `Publisher<?>` impl or target handler
is a `ReactiveMessageHandler`
* Add `@Reactive[] reactive()` attribute to messaging annotations
* Add `ConsumerEndpointSpec.reactive()`
Both options point to the same `ConsumerEndpointFactoryBean.setReactiveCustomizer()`
making the target endpoint always as a `ReactiveStreamsConsumer` independently of
the input channel and target handler
* Use the `Function` to customize a source `Flux` from the channel
* Test and document a new feature
* * Fix links in docs
* * Fix `ReactiveStreamsTests`
* * Rework `reactive()` attribute of messaging annotations ot a single `@Reactive` value
with default as `@Reactive(ValueConstants.DEFAULT_NONE)`
* Fix language in docs
* Fix `MessagingAnnotationUtils.resolveAttribute()` to use `requiredType.isInstance()`
instead of comparing classes since annotation instances are `Proxy` at runtime
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