MDEV-40672: Pluggable Aggregate Function

[drrtuy]

Add pluggable aggregate function support

What

Implement MDEV-40672 by extending function plugins to provide aggregate functions through the standard Item_sum lifecycle. Support grouped aggregation, DISTINCT, window functions, native and pluggable data types, and safe plugin lifetime management.

Key changes

• Extend Plugin_function descriptors to distinguish scalar and aggregate functions.
• Add Item_sum_plugin as the base class for plugin-provided aggregates.
• Support DISTINCT argument replay and pluggable result types such as UUID.
• Enable plugin aggregates as window functions, including moving-frame removal.
• Add test aggregates and MTR coverage for grouping, windows, prepared statements, plugin unloading, invalid descriptors, and type preservation.

How to test

Run:

/git/BuildOf_mdb-13/mysql-test/mtr function_plugin function_plugin_extra

Both tests pass.