[sophos-plugin] Implement Dependency Injection and remove global state usage

[osmontero]

The sophos plugin relies heavily on global state and lacks a proper dependency injection mechanism:

1. Excessive Global State: The plugin uses global configuration variables, leading to side effects and testing difficulties.
2. Lack of DI: Handlers and SDK clients are instantiated directly within functions, making it impossible to inject mocks for testing.

Proposed Changes:

• Move towards an instance-based configuration manager.
• Implement dependency injection for the Sophos API client and other dependencies.
• Ensure all components receive their dependencies explicitly.