[RAC][Rule Registry] Add tests for RuleDataService and RuleDataClient

[banderror]

Parent ticket: #101016

Summary

The indexing logic of Rule Registry (RuleDataService and RuleDataClient) is not covered with tests, not well tested and its quality and stability is unknown.

We need to cover it with integration and unit tests, checking:

• initial index bootstrapping
• subsequent index bootstrapping with the same parameters
• subsequent index bootstrapping with changed parameters (new mappings added/removed, settings changed, custom ILM policy set, etc)
• simulated race conditions between "kibana instances" bootstrapping the same index
• simulated race conditions between "plugins/solutions" bootstrapping different indices
• simulated race conditions between "kibana instances" bootstrapping multiple indices
• error handling of responses from Elasticsearch
• error handling of network errors
• error handling in data processing (our own code)
• writing a lot of documents to a single index
• writing a lot of documents to multiple indices
• load on ES due to many calls to getWriter or getReader
• etc, think of anything else

[elasticmachine]

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

[elasticmachine]

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

[banderror]

Hey everyone, I removed this ticket from the backlog of the Detection Rules area.

We (@elastic/security-detections-response-rules) are not the owners anymore (however feel free to still ping us if you have any tech questions about the ticket). Ownership of this ticket and other tickets related to rule_registry (like #101016) now goes to the Detection Alerts area (Team:Detection Alerts label). Please ping @peluja1012 and @marshallmain if you have any questions.