Category filtering for NordVPN

[AdamHebby]

Aims to fix #1643

I am not a GO developer, please read this code carefully!

After talking to NordVPN support, as long as a server supports P2P, a P2P connection will be established once it's determined there is a need for it (IE seeing P2P traffic). There's no way of knowing if it has connected to the P2P server other than contacting support with the IP address or results from curl https://ipleak.net/json/. I did ask if there was a way of starting the connection and specifying P2P from the get-go to avoid any switching but they said there was no documentation on this.

I've also seen online that this switch from standard to P2P can result in you being reconnected to a "better" server, but this could also introduce latency.

TL;DR Nord claims it "just works"

Also unsure on testing? I've added a single test case for the categories - please let me know if there's anything more I can do here.

[AdamHebby]

@qdm12 Can I get your review on this please? Also I saw you were concerned about the potential file size increase for servers.json, it's rather minimal at .9MB increase.

[qdm12]

Nice work 👍 💯

I left a few comments, the most important one being:

If the group can only be legacy_standard or legacy_p2p, would it be reasonable instead to just set the PortForward server model boolean to true when encountering legacy_p2p and not have categories at all?

That of course removes a lot of the fun in this PR, but it does make sense so far to me; even if there are other categories, I'm not certain there would be a usage to filter by category except for p2p? 🤔

[qdm12]

Sorry for the delay re-reviewing, here are some answers on previous conversations! 👍

[qdm12]

I've rebased and reworked your PR, just big congrats on the code, it's quite good 💯 !

I'm just going through it renaming categories to groups since that's the terminology used in the nordvpn API.

[qdm12]

Actually never mind, checking more the response from https://api.nordvpn.com/v2/servers?limit=10

it shows Legacy category for anything not regions, so let's keep it with categories.