Implement the Cluster Model data provider for the WAGED rebalancer

[jiajunwang]

Please find the design doc here:
https://github.com/apache/helix/wiki/Design-Proposal---Weight-Aware-Globally-Even-Distribute-Rebalancer

This data provider will help to convert Helix cluster status data into the rebalancer readable Cluster Model object. The result will be used to support rebalance calculation.

AC:

• finish the implementation based on the design.
• add unit tests for the new data provider.