Add Aggregator function & processor

* Have all the required dependencies in class path which give us a `MessageGroupStore` implementation
* Disable all their auto-configurations in the `ExcludeStoresAutoConfigurationEnvironmentPostProcessor`
* Enable only those which are configured by the `aggregator.messageStoreType` property
* Implement a fully reactive function around an aggregator making an interaction via a pair of `FluxMessageChannel`
* Fix checkstyle issues
* Change the app generator maven plugin to new coordinates

function-dependencies/pom.xml

@@ -185,6 +185,11 @@
                 <artifactId>websocket-consumer</artifactId>
                 <version>${project.version}</version>
             </dependency>
+            <dependency>
+                <groupId>org.springframework.cloud.fn</groupId>
+                <artifactId>aggregator-function</artifactId>
+                <version>${project.version}</version>
+            </dependency>
             <dependency>
                 <groupId>org.springframework.cloud.fn</groupId>
                 <artifactId>filter-function</artifactId>