Support user defined and pluggable consumer selector

**Is your enhancement request related to a problem? Please describe.**
When we try to Introduce pulsar to our existing system, compatibility is our first consideration. Firstly, our production system uses jump consistent hash algorithm to select consumers, but pulsar uses range consistent hash algorithm natively. It's impossible for us to guarantee that the same node can consume data with the same routing key from pulsar and our existing system simultaneously. Secondly, our system requires no change in consumer selecting rules when one or more consumers disconnect temporarily(service upgrade.etc). As our service is locally stateful and will be updated frequently, constantly route-jitter is harmful for us.

**Describe the solution you'd like**
Support user defined and pluggable consumer selector, so we can manage consumers' behavior by ourself when dispath messages.

**Describe alternatives you've considered**
We have implemented the same routing algorithm and consumer management strategy as our existing system in broker. But this implementation will cause great trouble for us to update the version of puslar regularly in the future.
@codelipenghui