Add Support for API Key Provider

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

We're also interested in this feature for basically the same reason - I'll try tagging @kwhinnery-openai (please feel free to reroute!) who I saw active on some other PRs in this repo recently. Happy to think through a proposal for the interface of a credential provider if it's something y'all are open to.

As a followup (not required initially, can file a followup issue for this), we'd also be interested in making this pluggable so users in our organization don't have to pass the openai_api_key_provider every time, but instead could install a plugin package that automatically provides auth.

Thinking aloud, I imagine the precedence order for resolving auth in the client when doing something like

client = OpenAI(
    api_key=...,
    base_url="...,
    http_client=...,
    openai_api_key_provider=...,
)

could then look like:

[current behavior] Any authorization headers set in the http_client arg, if this arg is provided
[current behavior] The value of the api_key, if provided
[current behavior] OpenAI environment variables (OPENAI_API_KEY etc)
Auth explicitly specified in openai_api_key_provider
Auth automatically provided by plugins, if api_key, http_client, openai_api_key_provider are not passed

openai / openai-python