Add support for Message<Foo> in stream apps

The FunctionInspector needs to be able to distinguish between a
function of Flux<Foo> and a function of Flux<Message<Foo>>. Then
it can do the conversion a level below.

Only supports Message->Message or POJO->POJO (no mixtures), so there
is only one new method in FunctionInspector.

No support in the web endpoints yet. But it's probably not so hard
to add.

spring-cloud-function-context/pom.xml

@@ -28,6 +28,10 @@
 			<groupId>org.springframework</groupId>
 			<artifactId>spring-web</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework</groupId>
+			<artifactId>spring-messaging</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>com.fasterxml.jackson.core</groupId>
 			<artifactId>jackson-databind</artifactId>