[Feature Request] Add Client method for non blocking try-get-result

Is your feature request related to a problem? Please describe.

Normal getting result blocks until completion:

await workflowHandle.result()

Describe the solution you'd like

A method that makes a single request and throws/returns an error if no result. Not sure what to call it, maybe:

try {
  await workflowHandle.tryGettingResult()
} catch (e) {
  if (e instanceof WorkflowIncomplete) {
    ...
  }
}

Someone asked whether it's possible, and @cretz thinks it's a good idea.

Additional context

Workaround: Call raw GetWorkflowHistory w/ the filter type set to close event and long poll set to false, and then translate the completed event manually

Start with the workflowHandle.result() entry point and the raw GetWorkflowHistory workaround described in the issue. Clarify the method name and the exact incomplete-result behavior, then verify that a single non-blocking request distinguishes an incomplete workflow from a completed result across the relevant SDK behavior.