Add initial support for RSockets (#2902)

* Add initial support for RSockets

* Add `spring-integration-rsocket` module and respective dependencies
* Implement `RSocketOutboundGateway` based on the Spring Messaging
`RSocketRequester`.
This component supports dynamic RSocket properties via expressions
against request message.
to handle `Publisher` for requests, it must be present in the request
message `payload` instead of `FluxMessageChannel` upstream, since the
last one just flattens events to be handled in the `MessageHandler` one
by one.
The result `Mono` is subscribed downstream in the `FluxMessageChannel`
or directly by the `AbstractReplyProducingMessageHandler`.
If result is a `Flux` it is just wrapped into the `Mono` to be processed
downstream by end-user code.
The point is that these request/replies are volatile and live in the
particular context meanwhile a `FluxMessageChannel` is long living
publisher in the application context boundaries.
* The `RSocketOutboundGatewayIntegrationTests` is an adapted copy of
`RSocketClientToServerIntegrationTests` from Spring Messaging
* Add `doOnError()` into the `Flux` created in the
`AbstractMessageProducingHandler` for `Publisher` replies

* * Use singular for the `RSocket` term
* Use no-op `Consumer` for the `strategiesConfigurer` and
`factoryConfigurer` in the `RSocketOutboundGateway` and also
`Assert.notNull()` in the appropriate setters to avoid null check during
`RSocketRequester.builder()` initialization
* Use `TcpServer.create().port(0)` in the
`RSocketOutboundGatewayIntegrationTests` to allow to select free OS port
and bind into it.
The selected port is used later for client configuration in the
`RSocketOutboundGateway` bean definition

* * Change `RSocketOutboundGatewayIntegrationTests.PORT` to lower case

spring-integration-rsocket/src/test/resources/log4j2-test.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<Configuration status="WARN">
+	<Appenders>
+		<Console name="STDOUT" target="SYSTEM_OUT">
+			<PatternLayout pattern="%d %p [%t] [%c] - %m%n" />
+		</Console>
+	</Appenders>
+	<Loggers>
+		<Logger name="org.springframework" level="warn"/>
+		<Logger name="org.springframework.integration" level="warn"/>
+		<Logger name="org.springframework.integration.rsocket" level="warn"/>
+		<Root level="warn">
+			<AppenderRef ref="STDOUT" />
+		</Root>
+	</Loggers>
+</Configuration>