Define and execute One-time task in one place.

[corecanarias]

Hi,
I've been looking at different schedulers specially for One-time jobs. For this use case I would like to define the code of a task and execute it in the same place, similar to what JobRunr does.

For example, if I have a function that creates a new user and also sends an email, I would like to do something like (in kotlin):

fun createUser(username: String) {
   val pass = generatePass()
   val savedUSer = newUser(username, pass)
   ... some more business logic
   
   private val welcomeEmailTask = TaskDescriptor.of("send-welcome-email")
       .instance(savedUser.id)
       .scheduledTo(Instant.now())
   val task = Tasks.oneTime(welcomeEmailTask).execute { taskInstance, executionContext ->
         println("Sending email...")
   }

   schedulerClient.registerAndScheduleIfNotExists(welcomeEmailTask, task)
}

This method will find if the task descriptor does not exists and create it and then immediately after schedule the task.
To implement this we could add the taskResolver to the SchedulerClient which should be easy since it's already available in the SchedulerClient.build()

Then on the registerAndScheduleIfNotExists detect if the task is not defined, and define it in place:

public <T> boolean registerAndScheduleIfNotExists(TaskInstance<T> taskInstance, Task<T> task) {
   if (!taskResolver.isDefined(task)) {
       taskResolver.add(task)
   }
  
  boolean success =
      taskRepository.createIfNotExists(SchedulableInstance.of(taskInstance, executionTime));
  if (success) {
    schedulerListeners.onExecutionScheduled(taskInstance, executionTime);
  }
  return success;
}

I couldn't find any reason why this wouldn't work? I think I could work on a PR if you are open to the suggestion?

Thank you.