Enable pushdown of a few aggregation functions for ClickHouse Connector

[sylph-eu]

Description

Partially addresses #7100.

This PR enables a pushdown of the following aggregation functions: COUNT(DISTINCT) (except varchars), COVARIANCE, and CORRELATION.

Implementation of the rest of the functions have certain nuances that require additional efforts:

• Decimal implementation truncates digits outside of the scale, while Trino rounds them up.
• Statistical functions return NaN values on empty sets, while Trino expects NULL.

I'll have a look whether limitations above can be workarounded to improve pushdown "coverage".

Additional context and related issues

Continuous work on #7100.

Release notes

(x) Release notes are required, with the following suggested text:

# ClickHouse
* Add support for pushing down execution of the `count(distinct)`, `corr`, `covar_samp`, `covar_pop` functions to the underlying database. ({issue}`7100`)