:with-time argument for timestamp selectors

Frist of all: This is another really great package. Thank you for your hard work!

There is the :date selector, which gives you the options to group items with or without a date.
Correspondingly it would be great to have some kind of :hour (or any other name) selector to group by items with hour specifications.
Meaning ':hour t' would match Item 1 and ignore Item 2, while ':hour nil' would ignore Item 1 and match Item 2:
* Item 1
SCHEDULED: <2021-09-26 Sun 15:00>
* Item 2
SCHEDULED: <2021-09-26 Sun>

My solution is this predicate function, in case anybode else needs something similar for now.
```
(defun my-org-hour-specification-p (item)
(s-matches? "[0-9][0-9]?:[0-9][0-9]" item))

(let ((org-super-agenda-groups
'((:pred my-org-hour-specification-p))))
(org-agenda-list))
```

Still, in my opinion this would be a nice enhancement to have built in.