* Add more Kotlin DSL for sub-flows
* Introduce `KotlinSplitterEndpointSpec`, `KotlinEnricherSpec` &
`KotlinFilterEndpointSpec` to avoid an explicit use of `IntegrationFlow` usage
from Kotlin DSL
* Fix `AbstractKotlinRouterSpec` to extend `ConsumerEndpointSpec` for access
to more endpoint options as it is with Java DSL for similar specs
* Introduce a `KotlinIntegrationFlowDefinition.publishSubscribe()`
for a `BroadcastCapableChannel` and `vararg` of `KotlinIntegrationFlowDefinition` builders.
This allows us to not introduce a `BroadcastPublishSubscribeSpec` wrapper for Kotlin
and let us to remove `publishSubscribeChannel()` DSL methods for `PublishSubscribeSpec`
since it is covered with the `publishSubscribe(PublishSubscribeChannel())` and
respective set of sub-flows in Kotlin DSL as subscribers
* Use new Kotlin specs in the `KotlinIntegrationFlowDefinition` instead of their
Java variants
* * Mention `kotlin-dsl.adoc` in the ToC
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