* - Add method: editContext.AddValidationTask(FieldIdentifier f, Task t)
 *   It adds the task to a HashSet<Task> on both the FieldState and the EditContext,
 *   so we can easily get all the tasks for a given field and across the whole EditContext
 *   Also it awaits the task completion, and then regardless of outcome (success/fail/cancel),
 *   it removes the task from those hashsets.
 * - Add method: editContext.WhenAllValidationTasks()
 *   Add method: editContext.WhenAllValidationTasks(FieldIdentifier f)
 *   These return Task.WhenAll(hashSet.Values), or Task.Completed if there are none
 * - Optionally also add editContext.HasPendingValidationTasks()
 * - Add method: editContext.ValidateAsync() that awaits all the validation tasks then
 *   returns true if there are no validation messages, false otherwise
 * - Now a validation library can register tasks whenever it starts an async validation process,
 *   can cancel them if it wants, and can still issue ValidationResultsChanged notifications when
 *   each task completes. So a UI can determine whether to show "pending" state on a per-field
 *   and per-form basis, and will re-render as each field's results arrive.
 * - Note: it's unclear why we'd need WhenAllValidationTasks(FieldIdentifier) (i.e., per-field),
 *   since you wouldn't "await" this to get per-field updates (rather, you'd use ValidationResultsChanged).
 *   Maybe WhenAllValidationTasks can be private, and only called by ValidateAsync. We just expose
 *   public HasPendingValidationTasks (per-field and per-edit-context).