[Security Solution] Customized rule alert telemetry RFC (elastic#230856)

**Partially addresses:
elastic/security-team#12507 (internal)

## Summary

RFC for the alert telemetry effort related to customized rule alerts
([internal
ticket](elastic/security-team#12507))

### Reviewers

As this RFC goes over modifications to the rule schema, I'm seeking
comments from all relevant teams and individuals

- @elastic/security-detection-rule-management 
- @elastic/security-detection-engine 
- @marshallmain 
- @approksiu 

### Timeline

~This PR was opened on **August 6th, 2025**, please take a look and
submit any comments by **August 13th** EOD~

RFC has been updated with suggestions and comments

Author: dplumlee