dialect/sql/schema: support setting schema name for migration (#4327)

dialect/sql/schema/postgres.go

@@ -30,6 +30,9 @@ type Postgres struct {
 // init loads the Postgres version from the database for later use in the migration process.
 // It returns an error if the server version is lower than v10.
 func (d *Postgres) init(ctx context.Context) error {
+	if d.version != "" {
+		return nil // already initialized.
+	}
 	rows := &sql.Rows{}
 	if err := d.Query(ctx, "SHOW server_version_num", []any{}, rows); err != nil {
 		return fmt.Errorf("querying server version %w", err)