Support batch offset fetch requests

[hyunseo0404]

Since OffsetFetch API version 8, it supports fetching offsets for multiple consumer groups in a single request as introduced in KIP-709.
It seems like Sarama currently does not support fetching offsets for multiple consumer groups (<= OffsetFetch API version 7).

We need to regularly fetch current offsets for multiple consumer groups for monitoring purposes, and it looks like this could help reduce the overhead of making a separate request for each consumer group every time.

Would it be possible to add support for the recent version of the OffsetFetch API?