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-deployer/src/main/java/org/springframework/cloud/function/deployer/FunctionExtractingFunctionCatalog.java

@@ -30,7 +30,8 @@ import org.springframework.cloud.function.context.FunctionInspector;
 import org.springframework.cloud.function.registry.FunctionCatalog;
 import org.springframework.util.MethodInvoker;
 
-public class FunctionExtractingFunctionCatalog implements FunctionCatalog, FunctionInspector {
+public class FunctionExtractingFunctionCatalog
+		implements FunctionCatalog, FunctionInspector {
 
 	private static Log logger = LogFactory
 			.getLog(FunctionExtractingFunctionCatalog.class);
@@ -65,6 +66,11 @@ public class FunctionExtractingFunctionCatalog implements FunctionCatalog, Funct
 		return (Supplier<T>) lookup(name, "lookupSupplier");
 	}
 
+	@Override
+	public boolean isMessage(String name) {
+		return (Boolean) inspect(name, "isMessage");
+	}
+
 	@Override
 	public Class<?> getInputType(String name) {
 		return (Class<?>) inspect(name, "getInputType");
@@ -112,14 +118,14 @@ public class FunctionExtractingFunctionCatalog implements FunctionCatalog, Funct
 		}
 		return invoke(FunctionInspector.class, method, arg);
 	}
-	
+
 	private Object lookup(String name, String method) {
 		if (logger.isDebugEnabled()) {
 			logger.debug("Looking up " + name + " with " + method);
 		}
 		return invoke(FunctionCatalog.class, method, name);
 	}
-	
+
 	private Object invoke(Class<?> type, String method, Object arg) {
 		for (String id : deployed) {
 			Object catalog = deployer.getBean(id, type);