feat: add lazy loading for data source file exports

[94ljy]

Description of change

Migration requires a file that exports dataSource.

Supports cases where dataSource is created asynchronously.

import { DataSource } from 'typeorm';

async function getDataSoruce(): Promise<DataSource> {
  const dataSource = new DataSource({
    // ...setting
  });

  return dataSource;
}

const dataSoruce = getDataSoruce();

export default dataSource; // type: Promise<DataSource>

Pull-Request Checklist

• Code is up-to-date with the master branch
• npm run format to apply prettier formatting
• npm run test passes with this change
• This pull request links relevant issues as Fixes #0000
• There are new or updated unit tests validating the change
• Documentation has been updated to reflect this change
• The new commits follow conventions explained in CONTRIBUTING.md