Add open-license energy balance sources

[Wegatriespython]

Refiling of #539 This is a draft PR corresponding to the working branch for the osce-energy-resilience project. It adds support to fetch UNSD and EUROSTAT data to provide an alternative to IEA for energy balance data.

Add tools/unsd.py and tools/eurostat.py: national energy balances from the UN Statistics Division and from Eurostat over SDMX, each as a registered ExoDataSource and as a load_data() function for consumers that use pandas and have no Computer. Both emit the same dimensionality and units as IEA_EWEB under their own key tags, so a consumer can swap providers without its n and y dimensions changing shape.

Add util.sdmx.fetch_data() for the part of the query that is specific to neither service.

Neither aggregation nor interpolation runs unless asked for, and absent observations are never filled. The UNSD balance carries nine fuel groups against the IEA product axis of sixty-nine codes, so it serves aggregate indicators and cannot drive a technology calibration;

How to review

na

PR checklist

- [ ] Continuous integration checks all ✅

• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
  - [ ] Update doc/whatsnew.

[codecov]

Codecov Report

❌ Patch coverage is 99.29078% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.5%. Comparing base (4f24a7e) to head (2da1998).

Files with missing lines	Patch %	Lines
message_ix_models/tools/eurostat.py	98.1%	1 Missing ⚠️
message_ix_models/tools/unsd.py	98.1%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #543     +/-   ##
=======================================
+ Coverage   74.3%   74.5%   +0.2%     
=======================================
  Files        323     327      +4     
  Lines      25850   26132    +282     
=======================================
+ Hits       19207   19488    +281     
- Misses      6643    6644      +1     

Files with missing lines	Coverage Δ
message_ix_models/tests/tools/test_eurostat.py	100.0% <100.0%> (ø)
message_ix_models/tests/tools/test_unsd.py	100.0% <100.0%> (ø)
message_ix_models/tests/util/test_sdmx.py	100.0% <100.0%> (ø)
message_ix_models/util/sdmx.py	98.6% <100.0%> (+<0.1%)	⬆️
message_ix_models/tools/eurostat.py	98.1% <98.1%> (ø)
message_ix_models/tools/unsd.py	98.1% <98.1%> (ø)

... and 1 file with indirect coverage changes

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