MDEV-34278: Implements DISTINCT for ST_Collect

[StefanoPetrilli]

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• [x] *The Jira issue number for this PR is: MDEV-34278

Description

This PR implements the DISTINCT clause for the ST_Collect function.

To ensure compatibility between the MySQL implementation of this function and the MariaDB implementation, the tests for this function have been cherry-picked from MySQL.

This patch has no side effects.

Release Notes

Adds the DISTINCT clause for ST_Collect` function.

How can this PR be tested?

An extensive test suite has been cherry-picked from MySQL.

Basing the PR against the correct MariaDB version

• [x] This is a new feature or a refactoring, and the PR is based against the latest MariaDB development 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

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