feat: Add OpenAI language model support

[aksg87]

Description

Add OpenAI language model support to LangExtract as an additional cloud-based LLM option alongside Gemini. The implementation maintains feature parity with existing backends. Implementation of schema constraints for OpenAI is pending.

Fixes #16

Feature

How Has This Been Tested?

$ python -m pytest tests/inference_test.py::TestOpenAILanguageModel -v
$ tox

All tests pass on Python 3.10 and 3.11. Also tested with live API calls and inspected HTML outputs on README examples.

Checklist:

• I have read and acknowledged Google's Open Source Code of conduct.
• I have read the Contributing page, and I either signed the Google Individual CLA or am covered by my company's Corporate CLA.
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
• I have made any needed documentation changes, or noted in the linked issue(s) that documentation elsewhere needs updating.
  • README documentation updated with OpenAI usage examples and configuration
• I have added tests, or I have ensured existing tests cover the changes
• I have followed Google's Python Style Guide and ran pylint over the affected code.