config: introduce sharding.rebalancer_mode

Closes #9657

@TarantoolBot document
Title: The `sharding.rebalancer_mode` option

The `sharding.rebalancer_mode` option can have one of three values:
`manual`, `auto` and `off`. Default value is `auto`.

If the option is set to `manual`, one of the replicasets must have the
`rebalancer` sharding role. The rebalancer will be in this replicaset.

If the option value is `auto` and there are no replicasets with the
sharding role `rebalancer`, the replicaset with rebalancer will
be selected automatically among all replicasets. If the value of the
parameter is `auto` and one of the replicasets has the sharding role
`rebalancer`, then the rebalancer will be in that replicaset.

If the option value is `off`, rebalancing will be disabled regardless of
whether a replicaset with the sharding role `rebalancer` exists or no
such replicaset exists.