Provider agnostic bitmask-vpn

Goal is to create a new UI page in the desktop app that'd allow users to configure a vpn provider by supplying its API url

The idea of the implementation:

1. start the desktop
2. choose from a provider (riseup, cylex), later: enter provider url
3. For each integrated provider, hardcode the API_URL in the desktop application
4. Fetch provider information from url
5. Save them in the desktop json configuration (+ which provider was chosen)
6. Make the Go code provider independent
7. After a restart of the desktop use the locally cached/saved provider information
8. Have a button in the settings for a "provider reset"

It should be possible to build the client provider agnostic and for a dedicated provider.