Extract function to generate migration

[lan17]

Description of change

Typeorm relies on a static DataSource being created and exported. However, this does not play nice with code bases where this is not desirable.

To support generating migrations in such code bases, I extracted the function to generate migration in such a way that you can just pass in instance of DataSource to it along with other options. In this manner, we should be able to use MigrationGenerateCommand.genMigration(...) without having to go through Typeorm CLI that assumes static 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

[lan17]

Updating tests should not be required since this is a simple refactoring change.

I did test the extracted function in my private codebase and it worked as expected.