feat: add generateWithOptionalTools to Kurt interface

[jemc]

Sometimes we may want to ask the LLM to produce a natural language response, but with the option of using some tools (in a structured data format) as part of its self-directed process of fulfilling the prompt.

This is a bit of a mixture of the prior two methods in the interface - we are expecting the LLM to make zero or more tool calls, and then eventually produce a natural language response.

See the README.md example and the spec examples to get a feel for how this works.

[jemc]

The example code in the README.md for this feature is a bit ugly, I know.

I want to follow up in a future PR to add a higher-level function you can call which wraps the generateWithOptionalTools function in a loop for you, and can call provided functions for you. Once I do that, I'll also update the README.md to show that nicer / more convenient style of example.

But @theoremonemehdi and I agreed together that we want to expose the "single call" version of this for apps that want full control, even if/when we add a future "repeated calls" function which wraps the loop for a more convenient interface.