Add support for PostgreSQL Audit Extension logs

# Integration release checklist

The PostgreSQL Audit Extension (or pgaudit) provides detailed session and/or object audit logging via the standard logging facility provided by PostgreSQL.
More info here https://github.com/pgaudit/pgaudit

### All changes

- [ ] Change follows the [contributing guidelines](https://github.com/elastic/integrations/blob/main/CONTRIBUTING.md)
- [ ] Supported versions of the monitoring target are documented
- [ ] Supported operating systems are documented (if applicable)
- [ ] Integration or [System tests](https://github.com/elastic/elastic-package/blob/master/docs/howto/system_testing.md) exist
- [ ] Documentation exists, [useful guidelines](https://github.com/elastic/integrations/blob/main/docs/documentation_guidelines.md) to follow
- [ ] Fields follow [ECS](https://github.com/elastic/ecs) and [naming conventions](https://www.elastic.co/guide/en/beats/devguide/master/event-conventions.html)
- [ ] At least a manual test with ES / Kibana / Agent has been performed.
- [ ] Required Kibana version set to: