Support sublabel in the calendar events plugin system

The sub-label feature can be useful in many aspects, like showing alternate calendar dates, holidays or emoji marks under the day numbers.

Bug report: https://bugs.kde.org/show_bug.cgi?id=429892

Discussions

1. Alternate calendar in plasma - feature request discussion
2. Making calendar decorations reusable between calendar implementations

Related MRs

Frameworks

1. !112 (merged)
2. https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/450

Feature

1. plasma/kdeplasma-addons!112 (merged)

Tasks

• Show Year-Month range of the alternate calendar besides the Gregorian month