add the ability to configure carmen imported countries

[BenMorganIO]

trafficstars

Adds the ability to filter which countries Carmen imports.

Checklist:

• [x] I have followed Pull Request guidelines
• [x] I have added a detailed description into each commit message
• [x] I have updated Guides and README accordingly to this change (if needed)
• [x] I have added tests to cover this change (if needed)
• [x] I have attached screenshots to this PR for visual changes (if needed)

[waiting-for-dev]

Thanks, @BenMorganIO! Do you think it'd make sense to use the already available .checkout_zone preference. Having another .available_countries setting is redundant.