Add fetch function for datasets from HarmBench

[romanlutz]

Is your feature request related to a problem? Please describe.

https://github.com/centerforaisafety/HarmBench/tree/main/data

There are several datasets including multimodal ones. We can't access them from PyRIT without additional work at this point.

Describe the solution you'd like

The goal here is to support fetching those datasets, similar to #267

Describe alternatives you've considered, if relevant

Additional context