config: allow to bootstrap anon replica from RO rs

This commit allows to bootstrap an anonymous replica from a replicaset,
where all the instances are in read-only mode.

The reason of the change is that there are no technical reasons to
forbid this action. An anonymous replica is not registered in `_cluster`
system space, so it can join a replicaset even if there are no writable
instances.

Fixes #9432

@TarantoolBot document
Title: config: anonymous replica is now supported

`replication.anon: true` option is now working.

There are configuration constraints that are related to anonymous
replicas.

* A replicaset must contain at least one non-anonymous instance.
* An anonymous replica can't be configured as writable instance using
  `database.mode` or `<replicaset>.leader` options.
* An anonymous replica can't be configured with
  `replication.election_mode` equals to `candidate`, `voter` or `manual`
  (only `off` is allowed).

A few more nuances about anonymous replicas:

* Anonymous replicas are filtered out from default upstream list.
* A `replication.failover: election` replicaset can contain anonymous
  replicas, but `replication.election_mode` defaults to `off` for them
  (unlike non-anonymous instances, where the default is `candidate`).
* `replication.failover: supervised` skips anonymous replicas, when
  choosing a bootstrap leader.
* A anonymous replica can joined a replicaset, which has all the
  instances in read-only mode (unlike a non-anonymous instance).

See details in [1] and [2].

[1]: https://github.com/tarantool/tarantool/issues/9432
[2]: https://github.com/tarantool/tarantool/pull/9418