[Journald] `host.hostname` is overwritten by the default `add_host_metadata` processor run in Filebeat

When using the journald input from Filebeat (currently used by the "Custom Journald logs" and the System integration once https://github.com/elastic/integrations/pull/11618), the field `host.hostname` can be overwritten by the `add_host_metadata` processor that runs by default on Beats running under Elastic-Agent.

The default processors added by the Elastic-Agent (and not shown in the policy/Fleet UI are):
```yaml
processors:
- add_host_metadata:
when.not.contains.tags: forwarded
- add_cloud_metadata: ~
- add_docker_metadata: ~
- add_kubernetes_metadata: ~
```

This is added by [this piece of code from Beats](https://github.com/elastic/beats/blob/ce4a17b3a960f779d61a6898f97080bbd2423033/x-pack/filebeat/cmd/root.go#L46-L64) when running under Elastic-Agent:
```go
func defaultProcessors() []mapstr.M {
// processors:
// - add_host_metadata:
// when.not.contains.tags: forwarded
// - add_cloud_metadata: ~
// - add_docker_metadata: ~
// - add_kubernetes_metadata: ~

return []mapstr.M{
{
"add_host_metadata": mapstr.M{
"when.not.contains.tags": "forwarded",
},
},
{"add_cloud_metadata": nil},
{"add_docker_metadata": nil},
{"add_kubernetes_metadata": nil},
}
}
```
That is later called during startup.

One way to avoid `host.hostname` being overwritten is to add the `forwarded` tag to the events produced by the journald input (or any input).

Currently the `forwarded` tag **is not added by default**.