A/B testing pass "variant" as argument to test data function

Allow dynamically creating set of test data based on variant input.

Data can be stored in external file on disk or remotely.

Example

evalite.each([
  { name: "GPT-4o mini", input: { model: "gpt-4o-mini", temp: 0.7 } },
  { name: "GPT-4o", input: { model: "gpt-4o", temp: 0.7 } },
])("Compare models", {
  data: async (variant) => {
    const data = getDynamicallyDataFor(variant);
    return data.map(it => { input: it.foo, expected: it.bar });
  },
  task: async (input, variant) => {
    return generateText({
      model: openai(variant.model),
      temperature: variant.temp,
      prompt: input,
    });
  },
  scorers: [Factuality, Levenshtein],
});

Start from the evalite.each example and inspect how its data and task callbacks currently receive arguments. The work is done when a variant can drive dynamic test-data creation and task execution, including data loaded from disk or remotely, with behavior covered by the relevant tests.