[Enhancement] Proton SRP Support

[kaplun]

Is your feature request related to a problem? Please describe.
Today, protonvpn-cli-ng relies on the user to enter the OpenVPN credentials stored in the user's Proton account, and then relies on all public unauthenticated APIs. However in the future these APIs are going to be protected (in order not to disseminate for free the list of IP addresses of ProtonVPN servers).

Describe the solution you'd like
The client needs to implement the Proton's flavor of the SRP authentication protocol and should thus switch to use Proton's account, and store locally only a refresh token.