Add parameter to set PRNG seed

[jemc]

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
• 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.