[Feature Request] Allow use of non OAI-like API url

Currently the benchmarker can only work with an endpoint accessible through an OAI-like API url set in `--url`. It would be great to extend this to any type of endpoint (e.g [Sagemaker endpoint](https://docs.aws.amazon.com/sagemaker/latest/APIReference/API_runtime_InvokeEndpoint.html)) and allow to set up a LLM client that can receive requests from the load tester.