Propagate local settings to background rebalancer

For the background rebalancer we don't propagate overridden settings yet and we should. The background rebalancer is started with `citus_rebalance_start`.

Two other issues are related to this:
- #6396 was the same for `rebalance_table_shards`
- #6357 is the same for `create_distributed_table_concurrently`

But because `citus_rebalance_start` doesn't use connections to itself some other approach should be done.