Function spawn 

pub fn spawn<A, T, F>(agent: A) -> AgentHandle<T>
where
    A: FnOnce(AgentContext<T>) -> F + Send + 'static,
    F: Future<Output = SageResult<T>> + Send,
    T: Send + 'static,

Spawn an agent and return a handle to it.

The agent will run asynchronously in a separate task.