Custom Record Grouping

We have a requirement to do some grouping based on
1. a timestamp in the **payload** (not current time of the kafka envelope time)
2. other custom fields in the payload

This is for Avro records going the GCS (for us) but clearly could be generalised

What I was thinking is to bypass the `RecordGroupingFactory` to allow for a custom record grouping to be defined, probably by a configuration parameter
So mostly a change to `AivenCommonConfig` to enable the parameter, `RecordGroupingFactory` to observe it

I am happy to submit a PR if this looks to be the "right" way to do this, but I am new to this library and happy to take a steer first