Add `JsonBytesToMap` function to allow conversion between byte[] and JSON Map.
Fixes https://github.com/spring-cloud/stream-applications/issues/441
* Introduce a `JsonBytesToMap` as a part of a `payload-converter-function` module
which is auto-discovered by Spring Cloud Function scanning algorithm - the `functions` package.
* Add a `payload-converter-function` as dependency into an `aggregator-function`
* Compose `jsonBytesToMap|aggregatorFunction` for the `aggregator-processor`
* Verify a `JsonBytesToMap` function in action with an `AggregatorProcessorTests`
* Mentioned such a payload conversion in the `aggregator-processor` README
