Add parameter to set PRNG seed

For LLM providers that allow setting the PRNG seed for sampling determinism reasons, Kurt should provide a way to do this.

Current expectations of support across LLM providers:
- OpenAI has this as a documented feature
- VertexAI Gemini doesn't appear to have it, but it may be an [undocumented feature](https://github.com/BerriAI/litellm/issues/2973#issuecomment-2065344363)
- Anthropic Claude doesn't appear to have it.
- Local LLMs should be able to easily support it.

For LLMs which do not support this feature, if the optional `seed` parameter is given by the user, Kurt should throw a `KurtFeatureIncompatibleError` (an error class which doesn't yet exist, but I've been thinking about for some time). We should also include a row for this in the #28 to highlight which LLM API providers have and don't have this feature.