feat: Allow installing release-candidates

[GeckoEidechse]

What feature would you like added?
For Northstar I prepare releases candidates before actual release in order to test all the proposed changes in a single update. These are published to Thunderstore and can be installed by players to gather feedback surrounding features and bugs before the actual release.
It would be nice if these release candidates could also be installed via Viper

Why should this feature be added?
Among mod-managers, Viper makes up around 1/3 of Northstar playerbase (the remainder being more or less equally split between FlightCore and VTOL).
By bringing release-candidate installation to Viper we can increase the percentage of players that have easy access to release-candidates and as such increase the amount of players that can test them :D

Additional Info
Both FlightCore and VTOL already support this. It would be nice to get Viper to feature parity ^^

Requires #142 as release candidates are only published to Thunderstore. With #142 implemented this feature request would be simply a toggle to switch from the northstar-Northstar package to northstar-NorthstarReleaseCandidate.

[0neGal]

I'll consider having a look at this and #142, and simply solve both in one PR, whenever I have time available, should be easy to do however, I really just need to make update.northstar() more agnostic when it comes to the downloading and extracting...