Proper support for global processor overrides

at this time we can specify global processors in controller by using elastic-agent config (fleet.yml) with fleet_override enabled.

If we specify global processor fleet.yml contains
```yml
processors:
- list

outputs.processors: [proc1]
```

issue is these values will overwrite any other processors or output processors defined by user in fleet.
Idea is to append global ones defined in fleet yml just after policy defined collection

Start with the fleet_override handling for processors in fleet.yml and trace how policy-defined collections are combined with global processors. Verify the current behavior for processors and outputs.processors, then define completion as appending the global entries after policy-defined ones without overwriting user values.

Written by the indexing model from the issue text.