Async validation enhancements

[SteveSandersonMS]

Follows on from #7614

Design ideas:

 * - 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).

Will implement this shortly after getting more of the system in place, assuming it still appears to be the correct design.

[mkArtakMSFT]

@SteveSandersonMS, how important is this to be done in 3.0? Can we do this post 3.0 without breaking customers?

[SteveSandersonMS]

@mkArtakMSFT Yes. Until we make built-in support, it would be extra work for developers to track the set of pending tasks associated with form validation so they can block submission until they are all done, but it is technically possible to do it.