MDEV-24943: Implement FILTER clause support for aggregate functions

[KhaledR57]

• The Jira issue number for this PR is: MDEV-24943

Description

Aggregates lacked the SQL-standard FILTER clause, forcing CASE-based workarounds that reduced readability.

This update introduces the ability to specify a FILTER clause for aggregate functions, allowing for more granular control over which rows are included in the aggregation. Also, improves standards compliance and makes queries clearer and more readable.

<aggregate_function> ( <expression> ) [ FILTER ( WHERE <condition> ) ] [ OVER ( <window_spec> ) ]

The <condition> may contain any expression allowed in regular WHERE clauses, except subqueries, window functions, and outer references.

Example:

-- Using FILTER clause
SELECT AVG(value) FILTER (WHERE status = 'active') FROM sales;

-- Equivalent using CASE
SELECT AVG(CASE WHEN status = 'active' THEN value END) FROM sales;

How can this PR be tested?

Running the Test Suite

./mtr aggregates-filter

Validating CASE vs FILTER Equivalence

Use the compare.py script to verify CASE and FILTER produce identical results:

python3 compare.py aggregates-filter-case.result

Expected Result File Format:

The script expects result files with this structure:

#
# Test N: Description
#
# 1. CASE version
column1	column2	column3
value1	value2	value3
# 2. FILTER version
column1	column2	column3
value1	value2	value3

The script identifies test headers (# Test N: Description), CASE/FILTER markers (# 1. CASE version / # 2. FILTER version), and compares the data rows that follow each marker.

Basing the PR against the correct MariaDB version

• This is a new feature or a refactoring, and the PR is based against the main branch.
• This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.