Ability to get full conversation history from a session

[domenic]

Right now it's expected that the web developer keep track of previous inputs and outputs themselves; they can only get the "next" output from the model.

However, it might be useful to allow retrieving the full session-so-far from the model. An example API:

const history = await session.history();

// returns an array of { role, content } pairs

This overlaps as a feature request with #50, so if there are independent use cases besides saving to disk or transferring across contexts, please comment with them here.