Add Hangar platform support

[Nlkomaru]

Summary

Add support for installing plugins from Hangar (PaperMC official plugin repository).

Current State

• GitHub ✅
• Modrinth ✅
• SpigotMC ✅
• Hangar ❌ (Not implemented)

Implementation

• Create HangarDownloader extending AbstractPluginDownloader
• API: https://api.hangar.papermc.io/
• URL pattern: https://hangar.papermc.io/{author}/{project}