(feat): add Faraday proxy support

[danilrwx]

Summary

Adding the ability to use a proxy when proxy from ENV is not suitable.

proxy = {
  :uri      => 'http://proxy.example.com',
  :user     => 'foo',
  :password => 'bar'
}

OpenAI::Client.new(access_token: "access_token_goes_here", proxy: proxy)

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