Add quantileExactLow and quantileExactHigh Implementations with Aliases for medianExactLow and medianExactHigh

Why it matters

These functions provide exact computations for low and high quantiles, improving accuracy for statistical analysis where precise boundary quantiles are required. This helps users retrieve more reliable quantile estimates, especially around median and tail values.

How to use it

Use the new aggregate functions quantileExactLow and quantileExactHigh in your SELECT queries to calculate precise low and high quantiles respectively. The aliases medianExactLow and medianExactHigh can be used as alternative names for median-specific calculations. For example:

SELECT quantileExactLow(value, 0.25) AS q1_low, quantileExactHigh(value, 0.75) AS q3_high
FROM table