Add support for DeleteOffsets operation

[faillefer]

Implement #1912
Add support for KIP-496: Administrative API to delete consumer offsets:
https://cwiki.apache.org/confluence/display/KAFKA/KIP-496%3A+Administrative+API+to+delete+consumer+offsets

[faillefer]

Waiting to sign the CLA
Submitting the form returns this error :

To sign our CLA 1 field needs to be changed
Submission rate limit exceeded. Please try again later

[dnwe]

@faillefer many thanks for this well structured PR, it is much appreciated! I've looked over the changes and they seem good to me, I've triggered the CI run and will double check the implementation against the protocol definitions, but I think it will be good to merge after that.

Please could you have another go at completing the CLA signing process today? Perhaps the service was under load when you last attempted.

[faillefer]

Thanks for your review
Sorry for the fmt issues. They are fixed by the last commit (https://github.com/faillefer/sarama/actions/runs/1213257314)

[faillefer]

I have retried to submit the CLA but i still have the same error.
I tried from home / office / phone in the last couple of days and it always return the same error

[faillefer]

@dnwe i was able to sign the CLA this morning

[bai]

Thanks for contributing!