Custom Record Grouping

[mkeskells]

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

[mkeskells]

for the timestamp source - something like #298

Its missing tests, documentation etc, and the extractor for a data path but should be an indication of the way that I am thinking

I added several timestamp sources, maybe more than we need, but once the door is opened ...

[mkeskells]

for the custom grouper draft PR is #300
I have added some tests and check it works for our use case next week, but would appreciate any feedback or re-steer