Provide API for country code list

I'm happy to review any provided MR's or patches. Could you elaborate on why you would need it?

For our project, we have a potential requirement to have a phone number form decoupled from Drupal, as well as the field provided by this module.

So we'd like both forms to use the same list.

Perhaps not a very common requirement...

Created a patch for listing the countries. Please review.

@sakthi_dev thank you for your contribution to this issue. I would like to highlight that you should use Drupal Best Practises and Coding Standards https://www.drupal.org/docs/develop/standards →

And I would like to have the API endpoint not behind and admin path? @malcomio do you have any suggestions?

Also if we're going to provide an API to expose the country list, we should implement the getting of the list as a service and re-use the service for the fields and other places where the country list from the lib is used.

After discussion this on DDD Vienna, my suggestion for a path would be /api/phone-number/country-options

And there is already a service providing the list, so please use that one to get the options.