perf(import): optimize search for domains claimed by other organizations (#8200)

# Which Problems Are Solved

Improve the performance of human imports by optimizing the query that
finds domains claimed by other organizations.

# How the Problems Are Solved

Use the fields search table introduced in
https://github.com/zitadel/zitadel/pull/8191 by storing each
organization domain as Object ID and the verified status as field value.

# Additional Changes

- Feature flag for this optimization

# Additional Context

- Performance improvements for import are evaluated and acted upon
internally at the moment

---------

Co-authored-by: adlerhurst <silvan.reusser@gmail.com>

proto/zitadel/feature/v2beta/feature.proto

@@ -60,4 +60,9 @@ enum ImprovedPerformance {
   IMPROVED_PERFORMANCE_PROJECT_GRANT = 2;
   IMPROVED_PERFORMANCE_PROJECT = 3;
   IMPROVED_PERFORMANCE_USER_GRANT = 4;
+
+  // Improve performance on write side when
+  // users are checked against verified domains
+  // from other organizations.
+  IMPROVED_PERFORMANCE_ORG_DOMAIN_VERIFIED = 5;
 }