Feature request: support filtering countries (JSON API)

[imWildCat]

Is your feature request related to a problem? Please describe.
As for the JSON API, filtering countries is a useful feature.

Describe the solution you'd like
Add an optional where() statement to the SQL query for the API of api/v1/proxies, its potential URL param is countries and it should support multi-countries in the query. In addition, only country code is supported, for example:

• CN
• US
• GB

Describe alternatives you've considered
None, at present.

Additional context
Add any other context or screenshots about the feature request here.