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

[fgbelidji]

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) and allow to set up a LLM client that can receive requests from the load tester.

[fgbelidji]

cc @Hugoch