Add manual build metadata generation outside of build process

[liortal53]

Summary

Refactored the build metadata generation logic to support manual generation outside of the build pipeline, while maintaining existing build-time generation functionality.

Key Changes

• Refactored BuildMetadataProcessor: Extracted the metadata generation logic into a reusable Generate(BuildTargetGroup, string) private method
• Added public Generate() method: Allows manual generation of build metadata using the active build target and a new GUID, without requiring an active build
• Created BuildMetadataMenu class: Added a new menu item "Tools/Build Metadata/Generate Now" to trigger manual metadata generation from the Unity Editor
• Maintained backward compatibility: The OnPreprocessBuild callback continues to work as before, now delegating to the refactored Generate method

Implementation Details

• The refactored Generate(BuildTargetGroup, string) method contains the core metadata generation logic previously in OnPreprocessBuild
• The public Generate() method uses EditorUserBuildSettings.activeBuildTarget to determine the build target group when called manually
• Each manual generation creates a new GUID, while build-time generation uses the actual build report's GUID
• The menu item is placed under "Tools/Build Metadata/" with priority 100 for easy access

https://claude.ai/code/session_011TzyEUutt99bHwmp8xacp1