[Alerting] Event Correlation Rules for o11y

[Erikg346]

Describe the feature:

Add Event Correlation Rules to Observability or make it available to all.

Describe a specific use case for the feature:
Observability needs a way to also correlate events across data sources. It's not just for Security:
For example,
Set up a condition for log.level: error in the logs-* index.
Set up another condition for system.filesystem.used.pct >= 80% in the metrics-* index.

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[cnasikas]

cc @elastic/obs-ux-management-team @jasonrhodes