refactor: pass a role string to OpenAI API

[anakin87]

Related Issues

While investigating on chat use cases, I discovered that we are sending requests like this to OpenAI:
[{'content': 'You are a helpful assistant.', 'role': <ChatRole.SYSTEM: 'system'>}]

instead of [{'content': 'You are a helpful assistant.', 'role': 'system'}]

Although this probably causes no problems, it is best to pass the correct format.

Proposed Changes:

• introduce a method to_openai_format in the ChatMessage dataclass
• use it in OpenAI generators and HF TGI Chat generator

How did you test it?

New tests, CI

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.
• I documented my code
• I ran pre-commit hooks and fixed any issue

[coveralls]

Pull Request Test Coverage Report for Build 8386997297

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 23 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.02%) to 89.259%

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Files with Coverage Reduction	New Missed Lines	%
components/generators/chat/hugging_face_tgi.py	2	94.85%
components/generators/openai.py	3	96.1%
components/generators/chat/openai.py	18	77.88%

Totals
Change from base Build 8376985254:	-0.02%
Covered Lines:	5410
Relevant Lines:	6061

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💛 - Coveralls

[vblagoje]

Nice catch thanks @anakin87