feat: support OpenAI-Organization for authentication

[anakin87]

Related Issues

• fixes Add OpenAI-Organization field to the request header #5210

Proposed Changes:

• Added the (optional) OpenAI-Organization string to headers for requests to OpenAI API
• In OpenAIInvocationLayer, OpenAIAnswerGenerator and EmbeddingRetriever

How did you test it?

New unit tests for each node involved.

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 5484968917

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 128 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+0.09%) to 44.744%

---

Files with Coverage Reduction	New Missed Lines	%
nodes/retriever/_openai_encoder.py	8	78.02%
nodes/prompt/invocation_layer/open_ai.py	10	80.85%
nodes/answer_generator/openai.py	14	85.0%
nodes/answer_generator/base.py	20	54.88%
nodes/retriever/web.py	23	71.85%
nodes/retriever/dense.py	53	26.45%

Totals
Change from base Build 5476747209:	0.09%
Covered Lines:	10309
Relevant Lines:	23040

---

💛 - Coveralls

[silvanocerza]

Awesome PR! 🚀