Add Lyrics Plugin System

Overview

This PR adds support for lyrics plugins, following the same architecture as audio source plugins. Lyrics functionality is now fully plugin-based, allowing the community to create providers for any lyrics service.

Changes

• Added lyrics to PluginAbilities enum
• Created lyrics endpoint interface in metadata plugin system
• Added Riverpod providers for lyrics functionality
• Comprehensive documentation for plugin developers

Benefits

• Community can create lyrics plugins for any service
• No app updates needed for new lyrics sources
• Consistent with existing plugin architecture
• Reduces main app size and complexity

Documentation

• LYRICS_PLUGIN_GUIDE.md - Complete development guide
• LYRICS_PLUGIN_TEMPLATE.md - Ready-to-use template
• LYRICS_PLUGIN_MIGRATION.md - Architecture overview

Example Plugins (to be created separately)

• spotube-plugin-lrclib-lyrics
• spotube-plugin-musixmatch-lyrics