Added strict_once mode to windowFunnel function to prevent duplicate event counts

Why it matters

The strict_once mode addresses the issue where the windowFunnel function could count the same event multiple times if it satisfies more than one condition. This improvement enhances accuracy in event sequence analysis by preventing duplicate counts of the same event.

How to use it

To enable this feature, use the windowFunnel aggregate function with the strict_once mode specified. For example:

SELECT windowFunnel('strict_once', time_column, condition1, condition2, ...) FROM table

This will count each event only once even if it matches multiple conditions.