Consider not saving chat history

[JamesHWade]

This is a similar thought to #29, but maybe a bit easier to implement. In some cases, we may want to make repeated API calls without saving the history for each call. An example is Edgar Ruiz's mall package.

Preserving the usage monitoring but removing the history would be nice. What about adding a save_history param to chat()?

[hadley]

For now, I'd recommend setting up a base chat object and then calling $clone() before you start chatting. I'm not sure what this interface will look like long term, but it's also likely to be connected to caching (#107)

[DavZim]

It would be great if we have something like this, that would make the user/developer experience a lot nicer imo:

library(ellmer)

# maybe save_chat or so is a better name ...
chat <- chat_openai(..., ephemeral = TRUE)

chat$set_ephemeral(TRUE)