Integration with Nexus Mods API for mod downloading

[mbround18]

Nexus mods has a api so you could setup a pairing and update your server mods with nexus and r2 mods.

https://app.swaggerhub.com/apis-docs/NexusMods/nexus-mods_public_api_params_in_form_data/1.0#/

Originally posted by @Kiaos in #1267