Refactor `GaiaWorkflow` to use `Context` for state management, add `PlanEvent` class, and enhance prompt handling in `agent.py`.

Update prompt templates and regex pattern to use tagged placeholders for better clarity

Add support for handling `additional_file` and `additional_file_path` in workflow agents, update `setup` method, and adapt prompt template accordingly

Update `setup` method to handle additional files with extensions, adapt prompt template for new variable

Add prompt templates for planning and formatted answering in gaia_solving_agent