Adding Structured output option to OpenAIGenerator

[TuanaCelik]

Proposed Changes:

Structured outputs are still a beta feature of OpenAI but they're super useful and I think we should provide an experimental extension to the already existent OpenAIGenerator. This PR adds this feature which calls .parse

How did you test it?

TBD

Notes for the reviewer

Can I have some help: I would like the model to default to gpt-4o-mini in the init...

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 10849979161

Details

• 16 of 34 (47.06%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.4%) to 93.53%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
haystack_experimental/components/generators/openai.py	14	32	43.75%

Totals
Change from base Build 10833606740:	-0.4%
Covered Lines:	3368
Relevant Lines:	3601

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💛 - Coveralls