[Security Solution] Adds customized_fields and has_base_version fields to internal rule schema (elastic#235394)

## Summary

Precursor to elastic#234793

**Must go into serverless before that PR is merged in**

Adds new fields related to [customized rule alert
telemetry](elastic#230856) to the
internal rule schema, an addition that must be done before adding them
to the security rule schemas (e.g. `RuleResponse`, etc.)

Author: dplumlee

x-pack/solutions/security/plugins/security_solution/server/lib/detection_engine/rule_schema/model/rule_schemas.ts

@@ -104,6 +104,14 @@ export const RuleSourceCamelCased = z.discriminatedUnion('type', [
   z.object({
     type: z.literal('external'),
     isCustomized: IsExternalRuleCustomized,
+    customizedFields: z
+      .array(
+        z.object({
+          fieldName: z.string(),
+        })
+      )
+      .optional(),
+    hasBaseVersion: z.boolean().optional(),
   }),
   z.object({
     type: z.literal('internal'),