Custom serializer for step/workflow output

Desired Behavior

Currently serializers are using JSON.stringify/JSON.parse. Any plans to support custom serializers to support non serializable json data? Just ran into a gotcha case where Date was serialized to string but still typed as a Date in step output.

Example Usage

const backend = BackendSqlite.connect(dbPath, { serializer: {
    serialize(data) {
        return devalue.stringify(data)
    },
    deserialize(data: string) {
        return devalue.parse(data)
    }
})

If there are no plans to support custom serializers, i guess it might be good to type StepOutputs in a way that makes typescript complain errors when output returned are not serializable

Maybe Output extends JsonValue or something

export interface StepApi {
  run: <Output extends JsonValue>(
    config: Readonly<StepFunctionConfig>,
    fn: StepFunction<Output>,
  ) => Promise<Output>;
}

Start with the BackendSqlite.connect serializer option and the StepApi output typing shown in the issue. Trace how step and workflow output is serialized, then define whether completion means supporting custom serialize/deserialize functions, restricting outputs to JsonValue, or both; verify the Date example and non-serializable output behavior.