sdk(python): add support for client_request_id -> X-Client-Request-Id

[arvindg-openai]

Changes being requested

• Forward any provided client_request_id as the X-Client-Request-Id header in _build_headers.
• Allow FinalRequestOptions/RequestOptions to carry the optional ID and thread it through make_request_options.
• Add unit coverage in tests/test_client.py for presence/absence of the header.
• Document usage in the README and bump the package version to 2.7.2.

Additional context & links

• Docs already reflect the new header
  https://platform.openai.com/docs/api-reference/supplying-your-own-request-id-with-x-client-request-id

Testing

• pytest -o addopts='' tests/test_client.py -k client_request_id

[arvindg-openai]

Looks like the examples GitHub Action tried to run examples/demo.py, but on forked repos that workflow doesn’t get the OPENAI_API_KEY secret. Without that key, the script sends an unauthenticated request and immediately hits a 401 (openai.AuthenticationError: "You didn't provide an API key"), so the job exits with code 1.