Add schema check for two factor authentication columns in migration file

[alibori]

Hi 👋

I propose this change in the migration file to avoid "existing columns" errors when installing laravel/fortify in an existing project.

Thank you very much.

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!