[FEATURE REQ] [OpenAI] Allow Forwards Compatibility for OpenAi Service version

[johnoliver]

As OpenAIserviceVersion is an ENUM, this blocks forwards compatibility with newer models created after after the latest azure-ai-openai release. Might be preferable for the serviceVersion builder to at least have the option of taking a string to allow forwards compatibility and moving to newer models without upgrading azure-ai-openai:

https://github.com/Azure/azure-sdk-for-java/blob/main/sdk/openai/azure-ai-openai/src/main/java/com/azure/ai/openai/OpenAIClientBuilder.java#L250