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_v2_human.go

@@ -434,15 +434,3 @@ func (c *Commands) userHumanWriteModel(ctx context.Context, userID string, profi
 	}
 	return writeModel, nil
 }
-
-func (c *Commands) orgDomainVerifiedWriteModel(ctx context.Context, domain string) (writeModel *OrgDomainVerifiedWriteModel, err error) {
-	ctx, span := tracing.NewSpan(ctx)
-	defer func() { span.EndWithError(err) }()
-
-	writeModel = NewOrgDomainVerifiedWriteModel(domain)
-	err = c.eventstore.FilterToQueryReducer(ctx, writeModel)
-	if err != nil {
-		return nil, err
-	}
-	return writeModel, nil
-}