Support :category in completion-extra-properties (bug#68214)

Support :category in completion-extra-properties (bug#68214)

* doc/lispref/minibuf.texi (Completion Variables): Add :category
to the table of completion-extra-properties.

* lisp/minibuffer.el (completion--metadata-get-1): New internal function.
(completion-metadata-get): Use 'completion--metadata-get-1'.
Thanks to Daniel Mendler <mail@daniel-mendler.de>.
(completion-extra-properties): Mention :category in the docstring.

* lisp/calendar/calendar.el (calendar-read-date): Use more
user-friendly let-binding of completion-extra-properties
with :category.