Makes common write_header function

[prabhatnagarajan]

AsyncEvaluator and Evaluator both use common code and functionality for generating a header in the scores.txt file. This PR unifies them.

• [x] Run an async agent with an evaluation and check it doesn't break
• [x] Run a normal agent with an evaluation and check it doesn't break
• [x] Run black

[prabhatnagarajan]

/test

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