Write query engine override param for generated queries

This allows them to be used in transactions and post-
migration callbacks.

moor/test/data/tables/todos.g.dart

@@ -1051,11 +1051,11 @@ abstract class _$TodoDb extends GeneratedDatabase {
     );
   }
 
-  Future<List<AllTodosWithCategoryResult>> allTodosWithCategory() {
-    return customSelect(
-            'SELECT t.*, c.id as catId, c."desc" as catDesc FROM todos t INNER JOIN categories c ON c.id = t.category',
-            variables: [])
-        .then((rows) => rows.map(_rowToAllTodosWithCategoryResult).toList());
+  Future<List<AllTodosWithCategoryResult>> allTodosWithCategory(
+      {QueryEngine operateOn}) {
+    return (operateOn ?? this).customSelect(
+        'SELECT t.*, c.id as catId, c."desc" as catDesc FROM todos t INNER JOIN categories c ON c.id = t.category',
+        variables: []).then((rows) => rows.map(_rowToAllTodosWithCategoryResult).toList());
   }
 
   Stream<List<AllTodosWithCategoryResult>> watchAllTodosWithCategory() {
@@ -1091,8 +1091,8 @@ mixin _$SomeDaoMixin on DatabaseAccessor<TodoDb> {
     );
   }
 
-  Future<List<TodoEntry>> todosForUser(int user) {
-    return customSelect(
+  Future<List<TodoEntry>> todosForUser(int user, {QueryEngine operateOn}) {
+    return (operateOn ?? this).customSelect(
         'SELECT t.* FROM todos t INNER JOIN shared_todos st ON st.todo = t.id INNER JOIN users u ON u.id = st.user WHERE u.id = :user',
         variables: [
           Variable.withInt(user),

moor_generator/lib/src/writer/query_writer.dart

@@ -49,7 +49,8 @@ class QueryWriter {
     _writeParameters(buffer);
     buffer
       ..write(') {\n')
-      ..write('return customSelect(${asDartLiteral(query.sql)},');
+      ..write('return (operateOn ?? this).') // use custom engine, if set
+      ..write('customSelect(${asDartLiteral(query.sql)},');
     _writeVariables(buffer);
     buffer
       ..write(')')
@@ -62,7 +63,10 @@ class QueryWriter {
     final upperQueryName = ReCase(query.name).pascalCase;
     buffer.write(
         'Stream<List<${_select.resultClassName}>> watch$upperQueryName(');
-    _writeParameters(buffer);
+    // don't supply an engine override parameter because select streams cannot
+    // be used in transaction or similar context, only on the main database
+    // engine.
+    _writeParameters(buffer, dontOverrideEngine: true);
     buffer
       ..write(') {\n')
       ..write('return customSelectStream(${asDartLiteral(query.sql)},');
@@ -77,12 +81,20 @@ class QueryWriter {
       ..write('\n}\n');
   }
 
-  void _writeParameters(StringBuffer buffer) {
+  void _writeParameters(StringBuffer buffer,
+      {bool dontOverrideEngine = false}) {
     final paramList = query.variables
         .map((v) => '${dartTypeNames[v.type]} ${v.dartParameterName}')
         .join(', ');
 
     buffer.write(paramList);
+
+    // write named optional parameter to configure the query engine used to
+    // execute the statement,
+    if (!dontOverrideEngine) {
+      if (query.variables.isNotEmpty) buffer.write(', ');
+      buffer.write('{QueryEngine operateOn}');
+    }
   }
 
   void _writeVariables(StringBuffer buffer) {