Implemented meanZTest Aggregate Function

Why it matters

The meanZTest function performs a statistical Z-test on means, enabling users to test hypotheses about the difference between sample means. This functionality provides valuable statistical analysis capabilities directly within ClickHouse, eliminating the need for external tools.

How to use it

Use the meanZTest function as an aggregate function in your SQL queries to perform a Z-test on sets of numerical values. For example, you can apply it within a SELECT statement combined with GROUP BY clauses to analyze grouped data statistically.