Add prompt_overrides to Crew

[gabriels1234]

This PR allows the Crew to receive prompt overrides for specific prompts (instead of one having to handle the entire json to the crew).
This proves useful for:

• Tweaking specific prompts for specific use cases.
• Easier to test many prompt optimization assumptions on-the-fly.

        crew = Crew(
            agents=agents,
            tasks=tasks,
            process=Process.hierarchical,  # Using hierarchical process
            manager_agent=manager,  # Assigning the manager agent
            verbose=True,
            prompt_overrides={
                "slices": {"format": "Some custom prompt"}}
        )

[gabriels1234]

Any help needed to move forward with this?
Thanks!