[Security Solution][Detections] Migrate from shadow saved object for managing actions configured to fire at specific intervals

[spong]

Currently the Detection Engine uses a few separate SO's for managing additional state, sometimes as a stopgap while other solutions were under development. Details on all SO's managed by Detections are detailed in this comment here: #60053 (comment).

Actions/Notifications SO's:

• Detection Actions/Notifications (Backed by Alerting/Actions SO)
• Detection Actions/Notifications (Shadow object of above SO for actions scheduled outside of rule execution time)

For managing Rule Actions that run at configured intervals (instead of at rule execution) the initial implementation included an additional shadow SO -- this issue is for investigating if it's possible to remove this additional shadow SO in favor of leveraging the functionality from the underlying alerting framework.

Custom Action Interval from Core Alerting UI:

[elasticmachine]

Pinging @elastic/security-detections-response (Team:Detections and Resp)

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)

[FrankHassanabad]

Other two discuss tickets:
#112327
#112209

[yctercero]

Addressed in 7.16 - related work found here.

Closing out for now.