Could we please extend the form type CountryType via an option to make it support ISO_3166-1 Alpha-3 country codes?

Sources for the country codes:

• https://www.iso.org/obp/ui/
• http://en.wikipedia.org/wiki/ISO_3166-1_alpha-3
• https://github.com/thephpleague/iso3166
• https://gist.github.com/brianherbert/1687655

I'm currently working on a project where I need country codes as 3 characters instead of just 2.

Would it be possible to include a new alpha3 option or something to support 3 characters country codes?

(This was first raised in #8492 but that issue was closed without being resolved properly)