Elastic Agent should allow a namespace to be configured

As a user I should be able to select to which namespace internal monitoring and log collection should be send.

```diff
agent:
  monitoring:
    enabled: true
+   namespace: test_1
    use_output: default
    logs: true
    metrics: true
```

based on the bug reporting from https://github.com/elastic/beats/issues/23020