Add Support for API Key Provider

[explocion]

Confirm this is a feature request for the Python library and not the underlying OpenAI API.

• [X] This is a feature request for the Python library

Describe the feature or improvement you're requesting

Similar to azure_ad_token_provider, support an openai_api_key_provider for OpenAI and AzureOpenAI instances. The application scenario is that OpenAI API keys are managed by some OAuth2 token servers, where a request is posted to the token servers and an API key with expiration is granted. In this case, for long running services, each time a request for OpenAI API is made, it is necessary to check and update the cached OpenAI API key. Therefore, I think it would be good to have an openai_api_key_provider to manage this situation.

Additional context

No response