Dynamic validation of styles in completion-category-overrides

Dynamic validation of styles in completion-category-overrides

The type of the defcustom completion-category-overrides must be able
to accommodate dynamic changes to completion-styles-alist, because
some packages (eglot) make their own additions.

This change fixes a failure in test-custom-opts.  See discussion at:
https://lists.gnu.org/archive/html/emacs-devel/2022-10/msg01969.html

* lisp/minibuffer.el (completion--styles-type):
Add an "Other" case that accepts any symbol which is then validated
dynamically against completion-styles-alist.