feat: restructure Unity packages into modular OpenFeature SDK and Confidence Provider

[nicklasl]

Summary

Restructures Unity integration from single unity-sdk into two modular packages for better separation of concerns.

Changes

• unity-openfeature-sdk: Pure OpenFeature implementation for Unity (com.unity.openfeature)
• unity-confidence-provider: Confidence platform integration (com.spotify.confidence.unity)
• Unity CI validation: New workflow for package compilation and structure validation
• Updated sample game: Now uses both packages separately
• Documentation: Added CLAUDE.md and individual package READMEs

Benefits

• Modular design allows using OpenFeature with other providers
• Clear separation between standard and platform-specific code
• GitHub distribution for both packages outside src/
• Independent versioning and evolution

Installation

{
  "dependencies": {
    "com.unity.openfeature": "https://github.com/spotify/confidence-sdk-dotnet.git?path=unity-openfeature-sdk",
    "com.spotify.confidence.unity": "https://github.com/spotify/confidence-sdk-dotnet.git?path=unity-confidence-provider"
  }
}

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