[SIEM integration] Standardize SQL information

As layed out in the [design doc](https://docs.google.com/document/d/1FoQRJkRQ1EhdZDGYnGNhCj4SKiY1quF9jII-XsWVOyA/edit#heading=h.jvh2iid8rap) information related to SQL queries could be interesting to integrate with the SIEM app. Required steps are:

- [ ] participate in defining SQL field set in ECS https://github.com/elastic/ecs/issues/514
- [ ] create mock ups for SIEM integration
- [ ] investigate if additional information should be collected by APM agents
- [ ] Failures in SQL queries result in a dedicated error event. Investigate if the error information should be duplicated in the span capturing the SQL information or if the separation into two events linking from the error to the span is fine.