Add merge Exchanges

[unitrium]

Issue

As mentioned on #5350, we also need a way of merging exchanges inside the new list datastructures as this is needed in the ENTSO-E parser at least. As it is looking at imports first and then merging with exports.

Description

• Adds a static method to ExchangeList to merge outputs together.
• Add a parent class MergeableList to hold common code between ProductionBreakdownList and ExchangeList.
• Add testing of Exchange merges.

Double check

• I have tested my parser changes locally with poetry run test_parser "zone_key"
• I have run pnpx prettier --write . and poetry run format to format my changes.

[VIKTORVAV99]

Bunch of small comments in regards to types plus some general questions and suggestions.

Will be nice to get this added!

[VIKTORVAV99]

I added a suggestion to change the docstring but beyond that this looks good to me.

I also took the liberty to format the files and fix a small spelling error. I hope you don't mind.

[unitrium]

As discussed with @mathilde-daugy offline we decided to rename it to AggregatableEventList.
Will merge it after her review.

[pierresegonne]

Looks better :)