config: introduce sharding role rebalancer

Part of #8862

@TarantoolBot document
Title: rebalancer sharding role in config

A new role for sharding was introduced - 'rebalancer'. This role can be
present at replicaset scope or above. A new change has also been made
to address this requirement: sharding.roles can now only be present in
replicaset scope or above.

There can be at most one replicaset with the rebalancer role.
Additionally, this replicaset must also have a storage role. If a
replicaset with this role exists, then the vshard rebalancer can only be
present in the replicaset with the rebalancer role.

If the rebalancer role is not specified, a rebalancer is selected
automatically from among the masters of the replicasets.