Allow creating calendars for (collections of) arbitrary contact attributes.

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Is your feature request related to a problem? Please describe.
When performing genealogy, searching for ordered contact attributes is impossible.

Describe the solution you'd like
Allow me to generate calendars which show the contact attributes I have selected, such as solely birthdays, or birthdays and deaths.

Describe alternatives you've considered
I don't know of any, but I've asked someone who discussed this 2 years ago at https://www.reddit.com/r/NextCloud/comments/pulfx8/comment/jziznw5/?utm_source=share&utm_medium=web2x&context=3) whether they've found a workaround.

Additional context
https://www.reddit.com/r/NextCloud/comments/pulfx8/is_there_a_way_to_get_more_automated_calendars/?utm_source=share&utm_medium=web2x&context=3

The issue does not identify files, tests, or an entry point. First locate the existing contact-attribute and calendar-generation implementations, then determine how selected attributes should be configured and represented. Done means users can generate calendars containing the chosen attributes, such as birthdays alone or birthdays and deaths.