Support display-sort-function in completion-category-overrides (bug#68214)

Support display-sort-function in completion-category-overrides (bug#68214)

* doc/lispref/minibuf.texi (Completion Variables):
Add 'display-sort-function' to the table of
'completion-category-overrides'.

* lisp/calendar/calendar.el (calendar-read-date): Add metadata
category 'calendar-month' for completing-read reading a month name.

* lisp/minibuffer.el (completion-category-defaults):
Add 'display-sort-function' with identity for the category 'calendar-month'.
(completion-category-overrides): Add customization for completion sorting
with 'display-sort-function' and a choice like in 'completions-sort'.
(completion-metadata-override-get): New function.
(minibuffer-completion-help): Use 'completion-metadata-override-get'
instead of 'completion-metadata-get' to get sort-fun from
'display-sort-function'.