Make LLM call optional when testing shields

[saichandrapandraju]

Currently, shield scan expects the shield(s) to be either at i/p or o/p to an LLM which makes LLM mandatory. This is an overhead if a user wants to only test shield(s). So we should introduce a flag like skip_llm=True to not consider LLM and allow only shield testing. This also requires to change the early exit logic since user wants all results from all shields.