feat: add default_headers for azure embeddings

[nuernber]

Related Issues

• fixes Support default_headers for AzureOpenAIDocumentEmbedder and AzureOpenAITextEmbedder #8534

Proposed Changes:

Follows the logic from AzureOpenAIGenerator for default_headers:

haystack/haystack/components/generators/azure.py

Line 71 in 852900d

default_headers: Optional[Dict[str, str]] = None,

and

haystack/haystack/components/generators/azure.py

Line 152 in 852900d

default_headers=self.default_headers,

How did you test it?

Ran the unit tests and also tested with my Azure set up that uses a subscription key.

Notes for the reviewer

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

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