v0.5.0

Refactor to make add new tools more easily.

Full Changelog: v0.4.0...v0.5.0

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Build

• build: Add nlohmann_json as a third-party dependency

CI

• ci: Conditionally find and include GTest in CMake
• ci: Add ctest step to GitHub Actions workflow

Docs

• docs: Add requirements section to README

Refactor

• refactor: Reorganize tool management with new structure
• refactor: Remove handler function in Tool class
• refactor: Move ReadFileTool to separate file
• refactor: Introduce ToolManager for tool handling

Tests

• test: Remove obsolete unit tests for tool management
• test: Reorganize includes in unit test files
• test: Add unit tests for ToolManager functionality
• test: Add unit tests for agent, message, and tools