Create a new api to completely drop a broker, including auto trigger rebalancing

```
curl -d 'disable' -X POST '/instances/Broker_XXX/state'
{"status":"Request to disable instance XXX is successful"}

curl -d 'drop' -X POST '/instances/Broker_XXX/state'
{"code":409,"error":"Failed to drop instance Broker_XXX - Instance Broker_XXX exists in ideal state for brokerResource"}
```

It's counter intuitive the broker instance is still in ideal state after disabling.

Start by tracing the broker instance state API described in the issue, including the disable and drop requests and the ideal-state validation for brokerResource. Determine where broker removal and rebalancing are handled. Done means a drop operation can remove a disabled broker from the ideal state and automatically trigger rebalancing, with the documented success and failure responses covered by tests.

Written by the indexing model from the issue text.