Add brokerSet id in kafka_cluster_state

[CCisGG]

As cruise control now supports BrokerSetAwareGoal (introduced in #1809), we may want to show the brokerSet id of each topic in the kafka_cluster_state.

Today, in kafka_cluster_state response, there is a section for broker state:

What we would like to have, is a new column BROKER_SET which shows the broker set id of each broker. The new broker state should look like below:

Preferably, this column should only show up when the BrokerSetAwareGoal is in the goals list.

This change will help the users to know what is the broker set id of each broker. Without this change, the broker set info will be opaque to users. E.g. after the rebalance happened, there is no way for users to know which broker set that each broker belongs to. This can introduce unnecessarily challenges for debugging cc and inspecting cluster status. Say if a user implements custom rules to assign broker set to brokers, the broker set arrangement would be unknown to the user after goal optimization.