KafkaSinkCluster - split DeleteGroups requests to ensure it is always routable.

[rukai]

If the DeleteGroups request contains groups that have coordinators on separate brokers shotover currently just routes it to the coordinator of the first group.
The correct way to handle this is to split it into multiple requests and then rejoin the responses back into a single response.

First double check that this is actually a problem by writing an integration test that deletes multiple groups at once, if we dont hit any routing errors then maybe we dont actually need to split these requests.

Then, refer to the other messages that are split such as fetch and produce copying their logic to split DeleteGroups requests and recombine the DeleteGroups responses.

[rukai]

I'm marking this as a good first issue since its largely copying logic that is already used elsewhere. But the splitting/combining logic is a bit complicated.

[rukai]

closed by #1785