Improve single-node cluster warning for discovery seed hosts only if discovery.seed_hosts is not set

[kingherc]

Description

PR #88013 introduces a periodic check that logs a warning that a fully-formed single-node cluster has discovery seed hosts set. For the moment, we ignore this warning in cases where the configured seed hosts are default addresses. However, we would like to improve that and ignore the warning only if discovery.seed_hosts is not set. Something like this:

// New:
// Case that the address of the machine is put in seed hosts. Should be ignored.
if (lastResolvedAddresses.size() == 1 && lastResolvedAddresses.get(0) == this.getLocalNode().getAddress()) {
        return;
}

if (DISCOVERY_SEED_HOSTS_SETTING.exists(settings) == false) { // new `if`
        Set<String> defaultAddresses = Set.copyOf(transportService.getDefaultSeedAddresses());
        if (lastResolvedAddresses.stream().allMatch(t -> defaultAddresses.contains(t.toString()))) {
            return;
        }
}

In order to do the above, however, we must first ensure that no user is putting a single-node address (e.g., 127.0.0.1) in the unicast_hosts.txt file, apart from maybe the node's address.

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[kingherc]

Finally went another route to only show the warning if the seed_hosts configuration is set. The warning will not apply to other discovery methods (e.g., file).