Add OffsetFetch and OffsetCommit

[Edgpaez]

First of all thank you for this package and the work you've put into it.

I have implemented OffsetCommit Request (Version 5) and OffsetFetch Request (Version 6). This is tested against Kafka 2.2.2.

This enables the tracking of:

• consumer groups
• when a new consumer first joins or a rebalance occurs.

I did not implement CollectClientMetrics for these requests since we're not using metrics at the moment.

Hope this is useful!

PS: Happy to modify this further if you think it's necessary 👍