Rebalance function workers

**Is your feature request related to a problem? Please describe.**
When we add a new pulsar function worker to the cluster (or a worker is restarted), pulsar does not rebalance the workers.

**Describe the solution you'd like**
Pulsar automatically rebalance function worker or there is an admin cli to rebalance the function worker

**Describe alternatives you've considered**
Recreate all functions

**Additional context**
Pulsar version: 2.8.1

Start by investigating how Pulsar function workers assign functions when a worker is added or restarted. Compare an automatic rebalance with an admin CLI operation, using the reported 2.8.1 behavior as the starting case. Done means functions can be redistributed without recreating all functions, with the behavior validated for both worker addition and restart.

Written by the indexing model from the issue text.