[FEAT] Add MultiAggJoiner, refactor AggJoiner

[TheooJ]

Sharing my WIP for issue #810

Adding the MultiAggJoiner also means refactoring the AggJoiner to match the Joiner, in particular:

• Adding a key argument
• Supporting polars in the test suite
• Only accepting a single table as an input
• Performing multiple small checks instead of one big check

Notes:

• For now, the check concerning duplicate column names happens after the aggregation in AggJoiner. Later we / I could move it before aggregation, I didn’t want to touch it for now since it is linked to the _polars.aggregate and _pandas.aggregate functions, let me know how you feel about this
• Should I change the landing page to show the multijoiners, or keep Joiner and AggJoiner ?

Also, thanks @jeromedockes & @Vincent-Maladiere for the discussions !

[jeromedockes]

LGTM, thanks a lot @TheooJ !

[glemaitre]

So this is a first pass. In general, I'm quite happy with what is there.

I assume that we will need an entry in the changelog as well.

[GaelVaroquaux]

You need a changelog entry for this (adding an entry in doc/CHANGES.rst)

[jeromedockes]

LGTM, thanks a lot @TheooJ !

@glemaitre , I think the main outstanding comment from your previous review was whether passing a list of strings (instead of list of list of strings) for the keys should be allowed, and that has been addressed.

[glemaitre]

LGTM. Thanks @TheooJ