fix: update typedef to support async version

[tuanddd]

• What kind of change does this PR introduce? This PR updates the AxMultiMetricFn type definition so that it can support asynchronous code

• What is the current behavior? AxMultiMetricFn only accept non-async function due to its return type Record<string, number>

• What is the new behavior (if this is a feature change)? Added Promise<Record<string, number>> as one of the return type so user can use the metricFn as asynchronous (LLM-judge, etc...)

• Other information:

  • Verified that all function invocations in src/ax/dsp/optimizer.ts correctly have await before

[tuanddd]

@dosco cheers

[tuanddd]

@dosco what's blocking this?