Support axios config

[Highnesslin]

Some regions require the use of a VPN to connect to GPT. I hope to open up Axios configuration to allow more people to use it.

like this

const agent = new HttpsProxyAgent('http://127.0.0.1:7890');

const model = createLanguageModel({
  OPENAI_API_KEY: 'API_KEY',
  OPENAI_MODEL: 'gpt-3.5-turbo',
}, {
  httpsAgent: agent,
});

[Highnesslin]

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[robgruen]

superseded by #382