[Security Solution] RFC for Prebuilt Rules Customization - Milestone 3 (

#171856)

Resolves: #171309

## Summary

- Creates an RFC for Milestone 3 of the Prebuilt Rules Customization,
including:
  - rule schema changes
  - mappings
  - migration strategy and technical implementation
  - exporting and importing rules
  - schema-related changes needed in endpoints
- calculation of `isCustomized` field on endpoints that update/patch
rules.
- additional changes needed to `/upgrade/_review` and
`/upgrade/_perform` endpoints
  - concrete diff algorithms
  - UI Changes

- Creates
`x-pack/plugins/security_solution/docs/rfcs/detection_response` folder
and adds it to CODEOWNER file, with owners the Detection Engine and Rule
Management teams.

.github/CODEOWNERS

@@ -1428,6 +1428,7 @@ x-pack/test/security_solution_cypress/cypress/tasks/expandable_flyout  @elastic/
 /x-pack/plugins/security_solution/common/detection_engine/rule_management @elastic/security-detection-rule-management
 
 /x-pack/test/security_solution_cypress/cypress/e2e/detection_response/rule_management @elastic/security-detection-rule-management
+/x-pack/plugins/security_solution/docs/rfcs/detection_response @elastic/security-detection-rule-management @elastic/security-detection-engine
 /x-pack/plugins/security_solution/docs/testing/test_plans/detection_response/prebuilt_rules @elastic/security-detection-rule-management
 /x-pack/plugins/security_solution/docs/testing/test_plans/detection_response/rule_management @elastic/security-detection-rule-management
 /x-pack/test/security_solution_api_integration/test_suites/detections_response/rules_management @elastic/security-detection-rule-management