Establish a criteria for when a country is available as an exit location

One of the features we're working on is the option to select a global exit location – i.e. the ability to route all of my circuits through exits in a specific country. In the future we'll probably make this feature more granular, but a single global setting will be sufficient for the MVP.

Here's an initial design exploration of the menu (just imagine the bottom-sheet scrolls):

However:

• It can't be a standard ISO list of countries, since not all countries have exits available.
• Similarly, we may want to require a minimum number of exits before a country is included in this menu – for security and speed reasons?
• Or, do we simply offer a manually curated shortlist of generally "healthy" exit countries for the MVP?

Whatever decision we go with, we might need to add some text somewhere that explains the options. Also, I'm posting this in the vpn project, but if it should go in Network Health (or elsewhere) instead, please feel free to move it.