Fixes https://github.com/spring-cloud/stream-applications/issues/275
Many functions make use of `ComponentCustomizer` ot let end-user
to provide any custom configuration for target component used by the function.
First of all for use-case which are not covered by standard configuration
properties and secondly for those options which cannot be represented
as a configuration option.
* Add a sentence to READMEs of those functions which make use of `ComponentCustomizer`
with an expected generic argument type
* Add README into `rabbit-supplier` where it is fully missed at the moment
* 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
