[Refactor] guidellm.benchmark shared pathway

[jaredoconnell]

We would benefit from a shared pathway for both the benchmark run and benchmark from-file pathways due to the new logic for outputs. Otherwise it won't support multiple output types, or the logic for selecting the output types in the new setup.

While doing so I would prefer to split up the benchmark_generative_test function since it is absolutely gigantic. I plan to reuse some of the code from that in from-file.