feat(catgrad-llm): tool calling

[georgewhewell]

currently we only support plain text inputs and outputs, even when using the structured conversion types

we need to:

• extend conversation types to accept function call inputs/outputs
• render them using model templates
• parse model output in order to return structured tool-call output blacks to user

subtleties:

• some models don't support function calling- probably we should error on templating in this case
• need to be careful parsing output- if we didnt call the model with any functions, we shouldnt render any function calls in the output. only functions passed to the renderer should exist in output