Perform true partial updates when updating the alerting rule after execution

[mikecote]

When an alerting rule finishes running, it updates select fields within the rule saved-object to reflect the new timestamp it last run, the success/failure outcome, etc. The saved-object update method recently got changed to support downward-compatible updates (#152807) by performing a get + Kibana side update before calling index. These extra requests when running many rules add extra latency and I/O to Elasticsearch that we do not need to perform when updating rules at the end of their execution.

Definition of Done

• Rules update themselves after a run by using the Elasticsearch client directly instead of the saved-objects client
• Only the necessary fields are passed to the update function to perform a partial update
• Preferably, other code using the partiallyUpdateRule function also leverage benefits from this work

[elasticmachine]

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