feat(@langchain/openai): support callable function for apiKey

[sinedied]

This PR adds support for passing a function to the apiKey property, as supported by the OpenAI SDK. The same feature was recently added to the Python version (langchain-ai/langchain#33532).

The AzureOpenAI extensions are not functionally impacted by the change, as they have a different way to handle this using azureADTokenProvider property.

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Latest commit: 3f4abea

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