[RAC][Rule Registry] Implement RuleDataService.getReader() that skips index bootstrapping

[banderror]

Parent ticket: #101016

Summary

Background: #108115 (comment)

The observability plugin initializes an instance of RuleDataClient in order to only be able to read from all observability indices (observability.logs, observability.apm, etc). It does this by calling:

    const ruleDataClient = ruleDataService.initializeIndex({
      feature: 'observability',
      registrationContext: 'observability',
      dataset: Dataset.alerts,
      componentTemplateRefs: [],
      componentTemplates: [],
      indexTemplate: {
        version: 0,
      },
    });

This doesn't lead to any index bootstrapping only because no component templates and other resources are specified. However, this is not super safe and it misuses this method.

We need to expose a separate method for getting a reader that would not even try to check if any bootstrapping is necessary:

    const ruleDataReader = ruleDataService.getReader({
      feature: 'observability',
      registrationContext: 'observability',
      dataset: Dataset.alerts,
    });

[elasticmachine]

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

[elasticmachine]

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

[weltenwort]

The RuleDataClient usage in the observability plugin has been removed in #109346, so from that point of view it's not required anymore.

[banderror]

Awesome, yeah, I can see it in the code 👍 Closing the ticket.