[Digital Clock] Add ability to set a custom date format string

Adds a new customDateFormat config key which is used when the dateFormat "StringEnum" is set to custom.
Shows a link to the Qt time formatting documentation next to the text field.
Qt doc link and text field are hidden when not set to custom date format.

CCBUG: 340982

Differential Revision: https://phabricator.kde.org/D18019