KAFKA-2687: Add support for ListGroups and DescribeGroup APIs

[hachikuji]

No description provided.

[hachikuji]

@junrao Ping for review. A few comments:

1. DescribeGroup in this initial patch only accepts one groupId. Should be straightforward to modify it to accept and return an array instead.
2. I haven't done so yet, but I think it would make sense to rename GroupMetadataRequest to GroupCoordinatorRequest.
3. I included an AdminClient to help with integration testing. This doesn't necessarily need to be included in this patch, but it seems like we'll need this functionality to implement ConsumerGroupCommand for the new consumer.

[junrao]

Maybe it's useful for an admin client to distinguish the two states?

[hachikuji]

@junrao Ready for another look.

[junrao]

Thanks for the patch. Other than making DescribeGroup take an array of groups, it looks good.

[hachikuji]

@junrao Added a couple commits to rename GroupMetadataRequest to GroupCoordinatorRequest and allow DescribeGroup to work with multiple groups.

[guozhangwang]

nit: ByteBuffer.allocate(0)