adding back compatibility layer

[soapergem]

Fixes #625. Adds back the compatibility layer that allows this library to be included via require "ruby/openai" so that it can cohabitate with OpenAI's official client library for Ruby. Please let me know if I missed anything here.

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[alexrudall]

Thanks @soapergem - looking into this, I believe using require "ruby/openai" should just work without this PR required. Did you already try that, if it doesn't work can I ask what error you get?

Edit: Thinking more about this, I guess the clash will still occur because even if you require with require ruby/openai, OpenAI::Client.new will still work for both gems. So we would need to figure out a way to force ruby-openai to only use Ruby::OpenAI in compatibility mode, if that's even possible. I wonder if reverting back to v3.2 would work for you, depending on which features you're using?