[feature][processing] Add a last_value function to the aggregate algorithm (Request in QGIS)

[qgis-bot]

Request for documentation

From pull request QGIS/qgis#42587 Author: @nirvn QGIS version: 3.20 (Feature)

** [feature][processing] Add a last_value function to the aggregate algorithm**

PR Description:

Description

This PR adds a new last_value aggregation method to processing's aggregate algorithm. It's an handy method to have when the dataset being aggregated has a meaningful ordering.

Commits tagged with [need-docs] or [FEATURE]

"[feature][processing] Add a last_value function to the aggregate algorithm"

[Gutzeit555]

I tried to fix it with qgis/QGIS#46211, but the attempt failed :-(