Add “real” multi-turn support to chat-enabled generators

[leondz]

Summary

Enable native support for chat histories in generators that support it

Basic example

When posting via APIs that support chat history (e.g. OpenAICompatible) we only send the current user prompt. We should be sending the entire chat history. Update generators to make this possible, adding a config flag (enabled by default) to send the full history as a list of dicts detailing actor and string.

Motivation

We must support multi-turn attacks

may subsume #202