FEAT: Adding simulated_conversation and adding prepended_conversation to context

[rlundeen2]

This PR adds simulated_conversation, which allows the creation of prepended_conversations using an adversarial chat. It also adds context of prepended_conversations to the adversarial conversations (which was lost before).

Early example usage might be something like:

• call generate_simulated_conversation_async with objective
• use this to set prepended_conversation and next_turn to Crescendo or TAP, making these attacks run much faster

One issue when we added prepended_conversation to multi turn is that we lost context for the adversarial_chat conversation. This PR adds that context back so the adversarial_chat can make better decisions.

Future work:

• Add generated_assistant role so we can better distinguish what actually comes from the objective target
• Update AttackExecutor to be able to run simulated_conversation. This will allow seamlessly doing things like simulating the first 4 crescendo turns.
• Generalize Fictional Scenario/Role play to wrap this (this also likely needs a few more changes to work well)

Tests:

• New unit tests added