rebalancer sharding role in config

[TarantoolBot]

Related dev. issue(s): tarantool/tarantool#8862

Product: Tarantool
Since: 3.0
Root document:

• https://www.tarantool.io/en/doc/latest/book/admin/vshard_admin/
• ???

SME: @ ImeevMA

Details

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.
Requested by @ImeevMA in tarantool/tarantool@171fd9c.