Kafka Admin Client should be able to manage user/client configurations for users and clients

Right now, Kafka Admin Client only allow users to change the configuration of brokers and topics. There are some use cases that users want to setOrUpdate quota configurations for users and clients through Kafka Admin Client. Without this new capability, users have to manually talk to zookeeper for this, which will pose other challenges for customers.

Considering we have already have the framework for the much complex brokers and topic configuration changes, it seems straightforward to add the support for the alterConfig and describeConfig for users and clients as well.