Add ability to subscribe to MDN Plus plans using USD card from an unsupported country

[bershanskiy]

I'm from a country not listed among supported countries](https://hacks.mozilla.org/2022/04/mdn-plus-now-available-in-more-markets/). I have a card in USD from a bank in my country and would like to use it to pay for MDN Plus as if I was in US. I tried creating a new Firefox account, but I can not manually choose a region.

Repro steps:

1. Be in a country not on this list
2. Go to https://developer.mozilla.org/en-US/plus#subscribe and see only Core/Free plan
3. Go to Firefox account settings page and observe no way to change region