frame-purpose is ineffective in ivy-switch-buffer

`ivy-switch-buffer` in ivy package ignores frame-purpose. That is, even if `frame-purpose-mode` is turned on and you are in a purpose-enabled frame, all buffers are displayed as candidates in the command. This seems to be because the function uses `'internal-complete-buffer` to produce a list of buffers:

```emacs-lisp
(defun ivy-switch-buffer ()
"Switch to another buffer."
(interactive)
(let ((this-command 'ivy-switch-buffer))
(ivy-read "Switch to buffer: " 'internal-complete-buffer
:matcher #'ivy--switch-buffer-matcher
:preselect (buffer-name (other-buffer (current-buffer)))
:action #'ivy--switch-buffer-action
:keymap ivy-switch-buffer-map
:caller 'ivy-switch-buffer)))
```

I am currently using a modified version of `helm-mini` for switching buffers, because I am mainly using EXWM and prefer selecting an EXWM buffer in a separate Helm source. Helm provides a better display of a buffer list. However, Ivy is faster, so I may switch to Ivy when I leave EXWM (which is likely to happen in the near future). Would you add support for `ivy-switch-buffer`?