chore: deprecate `to_openai_format` and create similar utility functions

[anakin87]

Related Issues

• part of #8144

Proposed Changes:

• deprecate to_openai_format
• create _convert_message_to_openai_format in components/generators/openai_utils.py
• create _convert_message_to_hfapic_format in components/generators/chat/hugging_face_api.py

How did you test it?

CI, new tests

Notes for the reviewer

I'm introducing some duplication, but it might make sense, considering how different formats can evolve.

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 10200746237

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 14 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.02%) to 90.111%

---

Files with Coverage Reduction	New Missed Lines	%
components/generators/chat/hugging_face_api.py	2	97.75%
components/generators/openai.py	3	96.43%
components/generators/chat/openai.py	9	79.28%
<!--	Total:	14		-->

Totals
Change from base Build 10200239554:	0.02%
Covered Lines:	6880
Relevant Lines:	7635

---

💛 - Coveralls