Add ConsumerGroupDescribe API

The goal of this task is to implement the ConsumerGroupDescribe API as described here; and to implement the related changes in the admin client as described here.

On the server side, this mainly requires the following steps:

1. The request/response schemas must be defined (see ListGroupsRequest/Response.json for an example);
2. Request/response classes must be defined (see ListGroupsRequest/Response.java for an example);
3. The API must be defined in KafkaApis (see KafkaApis#handleDescribeGroupsRequest for an example);
4. The GroupCoordinator interface (java file) must be extended for the new operations.
5. The new operation must be implemented in GroupCoordinatorService (new coordinator in Java) whereas the GroupCoordinatorAdapter (old coordinator in Scala) should just reject the request.

We could probably do 1) and 2) in one pull request and the remaining ones in another.

On the admin client side, this mainly requires the followings steps:

• Define all the new java classes as defined in the KIP.
• Add the new API to KafkaAdminClient class.