payments/db: split migration queries into dedicated SQLMigrationQueries interface

payments/db/migration1/migration_external_test.go

@@ -67,8 +67,14 @@ func TestMigrationWithExternalDB(t *testing.T) {
 		// Run migration in a transaction.
 		err := sqlStore.db.ExecTx(
 			ctx, sqldb.WriteTxOpt(), func(tx SQLQueries) error {
+				migTx, ok := tx.(SQLMigrationQueries)
+				if !ok {
+					return fmt.Errorf("db does not " +
+						"implement SQLMigrationQueries")
+				}
+
 				return MigratePaymentsKVToSQL(
-					ctx, kvBackend, tx, &SQLStoreConfig{
+					ctx, kvBackend, migTx, &SQLStoreConfig{
 						QueryCfg: sqlStore.cfg.QueryCfg,
 					},
 				)