Add auto-approval mechanism for optimization proposals

[ppatierno]

With the current interaction with Cruise Control, the user has to create a KafkaRebalance custom resource for getting an optimization proposal and after reviewing it, he has to manually approve by annotating with strimzi.io/rebalance=approve annotation.

It would be useful to have a way for an auto-approval by the operator. The user can decide that the proposal has to be approved automatically by the operator in any case or when, for example, some criteria are met maybe related to how many data are moved, how many replicas, balance score or specifying a time window when any rebalancing can happen. The criteria are something to investigate.