Allow modify task prompt content

[cuongpham-1001]

Currently, the task prompt content has been generated base on previous tasks output (combine previous context with current task's prompt)
Please allow us to modify it to optimize prompt. This will help on saving cost and performance
If possible, please allow us to hook on context generation process and modify it.

return Task(
            config=self.tasks_config["find_video"], 
            output_pydantic=Lesson,
            context_transform=self.transform_context_for_video
        )