feat: Adjust JSON structure to allow for other mod sources (but better)

[Alystrasz]

META PR, no verification needed (although it does touch verified-mods.json, so give it a double check nevertheless)

The idea here is to remove the dependency to Thunderstore and allow downloading mods from other platforms.

Closes #22.

TODOs

• Launcher PR (Adjust for restructured verified-mods.json NorthstarLauncher#748)
• CI update

[GeckoEidechse]

Schema change looks good. All links in verified-mods.json have been updated according to the new schema.

This schema adds support for external mod platforms by making the download links generic while also adding an enum to indicate the platform to the client to allow special handling.

[GeckoEidechse]

Just noticed we should also update the README for the new structure. Then it should be good to merge for real :D

[GeckoEidechse]

Requested changes have been addressed. Together with the previous review this is good to merge now :D