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

[nirvn]

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.

[github-actions]

@nirvn
This pull request has been tagged as requiring documentation.

A documentation ticket will be opened at https://github.com/qgis/QGIS-Documentation when this PR is merged.

Please update the description (not the comments) with helpful description and screenshot to help the work from documentors.
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Thank you!

[nyalldawson]

LGTM. Can we get a test too please?

[nirvn]

@nyalldawson , I couldn't spot a test for the aggregate alg. the begin with. Since the header indicates you're the author I had assumed you thought the underlying aggregate functions were well tested 😉

[nyalldawson]

I just ported from python -- I didn't write this one.

Isn't there tests in the standard processing yml test files?

[nirvn]

@nyalldawson , ohh, that's possible. I was looking for "native:aggregate", it's probably IDed as "qgis:aggregate".

[nirvn]

@nyalldawson , test added which revealed a bug in the code, so as always, good suggestion.

[github-actions]

@nirvn
A documentation ticket has been opened at qgis/QGIS-Documentation#6623
It is your responsibility to visit this ticket and add as much detail as possible for the documentation team to correctly document this change.
Thank you!