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>

internal/command/user.go

@@ -40,17 +40,8 @@ func (c *Commands) ChangeUsername(ctx context.Context, orgID, userID, userName s
 	if err != nil {
 		return nil, zerrors.ThrowPreconditionFailed(err, "COMMAND-38fnu", "Errors.Org.DomainPolicy.NotExisting")
 	}
-	if !domainPolicy.UserLoginMustBeDomain {
-		index := strings.LastIndex(userName, "@")
-		if index > 1 {
-			domainCheck := NewOrgDomainVerifiedWriteModel(userName[index+1:])
-			if err := c.eventstore.FilterToQueryReducer(ctx, domainCheck); err != nil {
-				return nil, err
-			}
-			if domainCheck.Verified && domainCheck.ResourceOwner != orgID {
-				return nil, zerrors.ThrowInvalidArgument(nil, "COMMAND-Di2ei", "Errors.User.DomainNotAllowedAsUsername")
-			}
-		}
+	if err = c.userValidateDomain(ctx, orgID, userName, domainPolicy.UserLoginMustBeDomain); err != nil {
+		return nil, err
 	}
 	userAgg := UserAggregateFromWriteModel(&existingUser.WriteModel)