Allow to pass your own httpx client (for example, to enable HTTP/2)

You may look into [OpenAI implementation](https://github.com/openai/openai-python/blob/b95be16e7c8a76c3d63335df13ab0d55ba3d5c35/src/openai/_base_client.py#L1335) to figure our how to keep default settings while allow users to pass their own client. For example, to enable HTTP/2