feat: add argentina_buenos_aires, handle multiple CSV files in pipeline

[yolile]

This was the implementation by Claude (with some review from my side)
But I don't think we can merge this because the CSV files are large, and the flatten tool consumed>60% of the available memory on the server when I tried it. I've killed the spider before it finished because the memory was getting higher and higher.

ref #1202

[codecov]

Codecov Report

❌ Patch coverage is 36.11111% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.75%. Comparing base (50b0aed) to head (4f60ae6).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
kingfisher_scrapy/pipelines.py	34.28%	23 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1255      +/-   ##
==========================================
- Coverage   91.88%   90.75%   -1.14%     
==========================================
  Files          29       29              
  Lines        1651     1687      +36     
==========================================
+ Hits         1517     1531      +14     
- Misses        134      156      +22     

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[jpmckinney]

How big / how many are the CSVs?

Flatten Tool is not memory efficient, that's for sure.

[yolile]

543M - award.csv
373M - contracts.csv
230M - parties.csv
568M - tender.csv