Added count for label, assignee, and milestone lists

What does this MR do?

Adds count for different board list types (label lists, assignee lists, and milestone lists).

What are the relevant issue numbers?

Closes #48800 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides