[Alerting][Discuss] Update handling of pre-defined IDs in rules client

[ymao1]

With this issue, we are deprecating support for pre-defined IDs from the HTTP rule create API but not the rules client. This allows internal users to continue creating rules with predefined IDs (although no internal user has taken advantage to date).

This issue is for discussing ways of allowing rules client consumers to specify pre-defined ID on rule creation and then being able to retrieve rules using that ID, even if the actual rule SO ID is different. This would allow consumers to create rules with the same pre-defined (static) ID in different spaces (since 8.0, SOs cannot have the same ID, even in different spaces).

Some suggestions to investigate:

• utilize tags (similar to what security solutions does now with internal tags)
• use the originId field in the saved object.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[gmmorris]

Marking as core services and technical debt as this directly relates to the Sharing of SO breaking changes in 8.0