Support config.storage as a failover stateboard.

[lenkis]

Product: Tarantool
Since: 3.3.0
Root document: https://www.tarantool.io/en/doc/latest/platform/replication/supervised_failover/
SME: @ georgiy-belyanin

Details

(This ticket is an effective copy of the original ticket from the deprecated EE repo: https://github.com/tarantool/enterprise_doc/issues/291)

Now a Tarantool config.storage cluster can be used as a stateboard for
the supervised failover.

In order to use config.storage as a stateboard, the config.storage
source should be configured and the cluster should be running. In such
configuration the failover coordinator uses config.storage as the
stateboard.

The supervised failover selects etcd/centralazied configuration storage
mode automatically based on the configuration. If both etcd and
config.storage sources are provided and available the first one is used
as a stateboard. Though, this configuration isn't recommended since etcd
and config.storage are interchangeable.

Requested by @georgiy-belyanin in https://github.com/tarantool/tarantool-ee/commit/6c8cfb00a014b67800853fa3c4f1e219be160187.