Adds routing keys in message headers

If the incoming message has a stream_routekey header, we use that
to route the message to a named function. Also adding the header
to messages coming out of suppliers.

The biggest change here is sort of orthogonal: it fixes a bug where
Spring Integration would subscribe twice to the same input channel
if the FunctionCatalog contains both functions and consumers. Then
when a message comes in it is dispatched to one or the other, but not
both. So the routing key couldn't work without fixing that
problem.

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/ContextFunctionCatalogAutoConfiguration.java

@@ -262,6 +262,9 @@ public class ContextFunctionCatalogAutoConfiguration {
 			else if (target instanceof Function) {
 				registration.target(target((Function<?, ?>) target, key));
 			}
+			for (String name : registration.getNames()) {
+				beans.put(name, key);
+			}
 			this.registrations.put(registration.getTarget(), key);
 		}